x PHYLUM ANNULATA 481 



horses and cattle while drinking. It attaches itself to the pharynx, 

 and may even descend the trachea. Others are permanent ecto- 

 parasites : for instance, Branchellion is found on the outer surface 

 of the Skate, Electric Ray, and other Fishes, entire families of the 

 leech, including individuals of all sizes, being sometimes found 

 crowded together on a small area of skin, which is distinctly 

 marked by their powerful posterior suckers. Other fish-parasites 

 are Pontobdella, on Rays, and Piscicola, on fresh-water Fish. 

 Aulostcma, to which, as well as to Ha-mopsis, the name Horse- 

 leech is applied, is carnivorous, feeding on snails and other 

 Mollusca ; so also are Clepsine, Nephelis, and the gigantic Macro- 

 bdella. The last-named genus and some others are of subter- 

 ranean habits, living in moist earth. The Land-leeches (Hceinadipsa) 

 live in the forests of many parts of 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 (Hasmadipsa) 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. Hitherto no 

 member of the class has been found in New Zealand, with the 

 exception of the marine Branchellion. 



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 Rhabdocceles 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 

 Gfunda (p. 241), the pseudo-metameres becoming converted into 

 true metameres by the development of inter-segmental constric- 

 tions and the completion of internal partitions. On the other 

 hand, it is deducible from the condition of a linear colony of 

 zooids proliferating at the posterior end, the zooids, though 



VOL. I II 



