Earthworms and Their Allies. Beddard Frank Evers

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СКАЧАТЬ aquatic life, are rendered possible by it. To drag such tender processes through stiff clay would surely break and tear them out. It may be also mentioned that among the aquatic families as a rule the intersegmental septa do not show that thickening in some of the anterior segments of the body which is so general a feature of the land-dwelling species. Finally it is only among the aquatic forms, and among them only in the families Aeolosomatidae and Naididae, that asexual reproduction by budding takes place. Indeed so common and usual is it in the genera of these families that even yet there are considerable lacunae in our knowledge of the organs of reproduction in the said families.

      Together with these general similarities are many points of structural difference among the worms inhabiting ponds, lakes, and rivers, which allow of their being divided into a number of quite distinct families.

      One of the most distinct of these families and lying in any case quite at the base of the series is the family Aeolosomatidae which includes a number of distinct species of delicate and transparent worms, and in whose integument are embedded round cells bearing a large brightly coloured oil drop; this is reddish or green in colour, or – and this more rarely – colourless, but still recognisable as an oil drop. The green sometimes even verges upon blue on the one side and yellow on the other, while the red may approach brown or purple. These worms are assigned for the most part to the genus Aeolosoma which is found in all of the great continents and of which seven or eight species are known. To a more doubtful genus Pleurophleps are assigned a few small worms which have the general appearance of Aeolosoma, but are without the coloured or colourless oil drops in the skin. These worms have a very large prostomium which is ciliated on the lower surface, and the body is not markedly segmented externally by constrictions or internally by septa. The bristles are slender and hair-like, but among them are in some species the shorter stouter bristles bifid at the free tip, which are so general among the aquatic families of the Oligochaeta. These worms are not uncommon objects in pools containing weed; and they are to be found usually crawling among the weed. They consist as a rule of but few segments to most of which a pair of nephridia belong. The ovaries and the spermaries are only known in a few forms and appear to be unpaired and lie respectively in the fifth and sixth segments. There are 1-3 pairs of spermathecae, and the sperm ducts if distinct from, are at least very like, nephridia. The ova appear to make their way to the exterior by a large aperture in the ventral middle line of a middle segment of the body. The vascular system contains uncoloured blood and is greatly simplified.

      The next family to the Aeolosomatidae in zoological position is clearly the Naididae. These are also small worms, but show in some respects a higher grade of organisation than their allies. While asexual generation is general, the reproductive organs are more commonly found than in Aeolosoma, though there are still many hiatus in our knowledge of the same in certain genera. Where they are known it has been found that the spermaries and ovaries are very far forward in the body, in the fifth and sixth segments respectively. The spermathecae are in segment six and the male ducts open into a terminal chamber, called 'atrium,' which is on the whole not unlike that of the Tubificidae. The blood in these worms is red as in the higher types, and thus differs from that of the genus Aeolosoma. The setae are rather varied, being in some cases long and slender, sometimes greatly exaggerated in length as in Ripistes; other setae are forked at the free end, and in Paranais this is the only kind of setae met with. A marked feature of this family is that the dorsal bundles of setae do not always begin like the ventral setae upon the second segment of the body. Indeed in Schmardaella there are no bundles of dorsal setae at all. The Indian genus Branchiodrilus is remarkable for the fact that it has paired processes of the body which may be termed gills and which in some segments involve the dorsal setae. Another kind of gill is found in the genus Dero (which has many species) and in the allied Aulophorus. These are placed round the vent, and are ciliated. Other genera are Nais, Chaetogaster, Vejdovskyella, Amphichaeta, Stylaria, Macrochaetina, Pristina, Naidium.

      Several genera, Pristina, Nais, Dero, are found in many parts of the world; but it is not possible at present to consider very seriously the facts of their geographical distribution.

      Next to the Naids a group of aquatic worms present themselves for consideration which are usually placed in three distinct families, which families are however rather hard to define. These three families are the Tubificidae, Phreodrilidae, and Lumbriculidae. The Phreodrilidae were at one time placed with the Tubificidae by Michaelsen. It is now perhaps the general opinion that they form a family of their own, at any rate since the discovery of two other genera Phreodriloides and Astacopsidrilus, besides the original genus founded by myself, and named Phreodrilus from the fact that the species was found in a deep well in New Zealand.

      The distribution of this family especially of the genus Phreodrilus is extremely interesting. The genus Phreodrilus is the only one genus of the aquatic Oligochaeta which has, like Notiodrilus, a circumpolar range, the pole being the south pole. It occurs in New Zealand, in Kerguelen, and, if we are to accept the opinions of Drs Michaelsen and Benham that my genus Hesperodrilus is to be merged in Phreodrilus, in Patagonia also.

      In this genus the male pores are upon the XIIth segment while the spermaries lie in segment XI. Moreover the spermathecae lie behind the male pores. In the Tubificidae on the other hand it is at least the rule for the spermaries and male pores to be pushed a segment further forwards. And the spermathecae are before the male pores. Phreodriloides is like Phreodrilus but has no spermathecae. It is also New Zealand in range. Astacopsidrilus is Australian and is semi-parasitic upon the Crayfish Astacopsis, whence its generic name. Phreodrilus branchiatus is one of the few forms of Oligochaeta that possesses gills. Of these there are a series of pairs on about the last eleven segments of the body. They are lateral in position, and thus contrast with the also gilled Branchiura sowerbii, where the gills, also on the posterior segments of the body, are more numerous and lie dorsally and ventrally, a pair to each segment.

      The Tubificidae differ from the Phreodrilidae mainly in the points already noted. There are a considerable number of genera of which the following are the best known, viz., Tubifex, Limnodrilus, Limnodriloides, Branchiura, Lophochaeta, Ilyodrilus, Psammoryctes, Clitellio, Telmatodrilus, Bothrioneuron, Lycodrilus.

      The Tubificidae are mainly northern temperate forms, and a few of them such as Clitellio arenarius and 'Peloryctes inquilina1' are found on the sea coast. There are also a few of this family in the southern hemisphere. These forms include Clitellio abjornseni from Australia, and a few species of Branchiura from New Zealand and the islands of the Antarctic ocean. There is also to be mentioned Rhizodrilus (or Vermiculus) aucklandicus from the island of that name in the New Zealand area. The only tropical species appears to be the Indian and Malayan Bothrioneuron iris, though this genus also occurs in Europe and in southern South America. It is quite likely however that Branchiura sowerbii, a species known at present from tanks in hot houses, may be a tropical American species.

      The family Lumbriculidae is yet more restricted in its range. It has not yet been met with away from the temperate northern hemisphere, and the great variety of species recently described from Lake Baikal by Dr Michaelsen is a very remarkable fact. The Lumbriculidae are entirely fresh water in habit and not even partially terrestrial. The following are the principal known genera: Lumbriculus, Trichodrilus, Rhynchelmis, Phreatothrix, Claparedilla, Stylodrilus, Mesoporodrilus, Sutroa, Eclipidrilus, Aurantina, Athecospermia, Lamprodrilus, Teleutoscolex.

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