x PHYLUM ANNULATA 523 



the world, and in spite of their small size, which does not exceed 

 30 mm. in length and 5 mm. in diameter, are much dreaded for 

 the persistent attacks they make on men and cattle. 



Many genera are very widely distributed : for instance, the 

 Land-leeches (Hceni adipsa) occur in India, Ceylon, the East 

 Indies, Japan, Australia, and South America, a distribution 

 which seems to indicate that the group is one of great antiquity. 



GENERAL REMARKS ON THE ANNULATA. 



A special feature of the Annulata, as distinguished from the 

 phyla previously dealt with, is metameric segmentation. In some 

 of the Platyhelminthes, as we have seen, there obtains a con- 

 dition to which the term pseudo-metamerism is applied. In such 

 cases there is a serial repetition of certain of the organs gonads, 

 diverticula of the intestine, nerve-commissures, &c. in such a 

 way as to produce a jointed appearance, though the body is not 

 divided into definite segments. An appearance resembling seg- 

 mentation is produced also in certain Rhabdocoeles that multiply 

 by budding, chains of zooids remaining connected together 

 for a time. In the strobila of the Cestodes we recognise a con- 

 dition which might be described as combining pseudo-metamerism 

 with the formation of a chain of zooids. The condition of true 

 metamerism, as we observe it in the Annulata, is capable of being 

 deduced from a condition of pseudo-metamerism as it occurs in 

 Gnnda (p. 255), the pseudo-metameres becoming converted into 

 true metameres by the development of inter-segmental constric- 

 tions and the completion of internal partitions. If we suppose that 

 during this process serially repeated outgrowths of the enteron 

 became separated off to form series of ccelomic sacs enclosing the 

 o-onads, a condition would be reached not far removed from that 



o 



which characterises the Annulata. On the other hand, the meta- 

 meric condition is deducible from the condition of a linear colony 

 of zooids proliferating at the posterior end, the zooids, though 

 becoming each complete in itself, not, under ordinary circumstances, 

 becoming detached. The establishment of a closer connection 

 between the corresponding organs of the zooids in such a colony, 

 with the special differentiation of the anterior end, would result in a 

 condition closely resembling the metamerism of the Annulata, It 

 is conceivable that a condition of pseudo-metamerism was followed 

 by that of a linear series, not of zooids, but of comparatively 

 independent parts capable of readily reproducing the animal when 

 detached, and that a secondary closer connection established 



