Earthworms and Their Allies

by Frank E. Beddard

Within the limits of the same sub-family as that which contains Notiodrilus, i.e. the Acanthodrilinae, the changes of structure affect all the principal organs of the body except the nervous system, but are not very large and vary from genus to genus. They are mainly perhaps in the direction of reduction and simplification. Thus in Chilota, Maheina and Yagansia the spermaries are reduced to one pair in either the Xth or XIth segment, while in Yagansia one pair of spermathecae and of spermiducal glands have also disappeared. In Microscolex the spermaries remain normal, but one pair of spermathecae and of spermiducal glands have disappeared, the remaining organs of these series being in the IXth and XVIIth segments respectively. In Microscolex, Chilota and Yagansia, moreover, there is a further degeneration in the disappearance of the calciferous glands. These glands are often absent and sometimes less developed in the New Zealand Maoridrilus, which is otherwise not a degenerate form and differs characteristically from Notiodrilus by the fact that the paired nephridia alternate in position in successive segments, being now in front of the dorsal, and in other segments in front of the ventral, pairs of setae. While these genera are somewhat degenerate, the New Zealand Plagiochaeta has undergone specialisation in an upward direction. For the setae of each segment are increased to a large number much exceeding eight.

It is not a long step to the sub-families Diplocardiinae and Trigastrinae. In the first of these, an American race confined to the northern and central parts of that continent, the male pore shows a tendency to move backwards, being situated on any of segments XVIII-XXI. The two spermiducal glands follow it, but are always placed one pair in front and one behind, as in Notiodrilus. In this group we get a new feature of specialisation in the duplication or triplication of the gizzard.

So too with the Trigastrinae where there are either two or three gizzards; but in this sub-family another modification has become apparent. The paired nephridia have disappeared and their place is taken by several, often quite numerous, pairs of much smaller nephridia called on that account 'micronephridia' instead of 'meganephridia.' To this sub-family belong the especially African but also American and Malayan Dichogaster, whose name is derived from the important fact that it possesses two gizzards.

Not far off is to be placed another sub-family, that of the Octochaetinae, which is New Zealand and Indian in range, the intermediate countries being, strange to say, not populated by this race of Oligochaeta. The group contains several genera of which Octochaetus, Eutyphoeus, and Dinodrilus are the best known. All these worms agree in the main features of their anatomy with Notiodrilus; but they have diverged in different directions. Thus in Octochaetus the typical two pairs of gonads and glands belonging to the generative system have been retained, while the nephridial system consists of micronephridia; in Eutyphoeus, one pair of spermiducal glands has disappeared, and as a general rule the species of this genus have only one pair of spermaries and the corresponding pair of sperm ducts. They are close to Octochaetus. The third genus mentioned, Dinodrilus, is a New Zealand form specialised in possessing 12 setae in each segment. Otherwise it is not far removed from Octochaetus.

Fig. 8 Ventral view of Eutyphoeus masoni. p papillae, ♂ male pores, ♀ oviduct pores. (× 3.)

A fifth sub-family is also easily referable to the type whose structure has been dealt with as a preliminary to the present survey. That sub-family is the Ocnerodrilinae which is American and African in range. These worms are somewhat degenerate in comparison with their allies. Thus the calciferous glands are reduced to a single pair or to a single gland in the IXth segment, the nephridia though regular and paired have no covering plexus of blood vessels, and the worms themselves are slender and delicate, being indeed often aquatic in habit. The spermiducal glands, which are as in the former sub-families independent of the sperm ducts though sometimes opening in common with them into a short pocket-like ingrowth of the skin, are reduced in their minute structure and much simpler than in the other types.

The genus Kerria is the least reduced perhaps. It has the male pores on segment XVIII with a pair of spermiducal glands on the segments preceding and following this in the typical Acanthodriline fashion. There are two pairs of spermathecae in VIII and IX, but the spermaries are reduced to one pair in X. The gizzard is present. Ocnerodrilus is a little further reduced from this last. The gizzard has gone; there is but one pair of spermiducal glands (as a rule) opening in common with the extremity of the male duct on to segment XVII; the spermathecae also are reduced to one pair, but there are two pairs of spermaries. The African Nannodrilus is more robustly built. There are two gizzards, the male duct opens into a muscular pouch, into which also open one of the two or three pairs of spermiducal glands. There are two or three other genera and sub-genera not showing any great divergencies from the range of structure indicated in briefly defining those enumerated above.

Finally, we have the largest of all the sub-families of this family, viz. that of the Megascolecinae. These worms are mainly tropical in range and also mainly found in the Indo-Australian portion of the world. In them the sperm ducts open in common with the usually single pair of spermiducal glands and prevalently upon the XVIIIth segment. The glands moreover have not always, though they often have, the tubular form shown in all the sub-families hitherto considered. In many forms they are branched and lobate glands, and if there are two pairs one may be of one type and the other of the second and derived type, as for instance in Megascolex ceylonicus. Furthermore, it is much commoner among the genera of this sub-family for the setae to become numerous and to spread right round the segment; this condition is seen in the genera Pheretima, Megascolex, Diporochaeta, Perionyx, Plionogaster. The spermathecae also are commonly more than the typical two pairs of the forms already considered, and in certain species (for example Pheretima hexatheca) there are as many as six pairs of those organs which are moreover—and in this they resemble the majority of species of the last sub-families—nearly always furnished with a diverticulum or diverticula. The nephridia are either paired or numerous and these various characters allow of the sub-family being split up into sixteen genera or thereabouts.

As an example of another type of organisation and as contrasting with Notiodrilus we may now briefly describe the structure of the genus Pontoscolex (better known as Urochaeta), a member of the American and African family Geoscolecidae.