ZOOLOGY AND BOTANY, MIOKOSCOPY, ETC. 951 



tlie first third contains the most important, and so it is always retained. 

 Although this disappearance must have been quite gradual phyloge- 

 netically, yet in ontogeny the result alone is seen, the reduced somite 

 being reduced in the embryo. On the other hand, all the shortenings, 

 reductions, and fusions of the rings, and the appearance characteristic 

 of the species or bearing on their sexual life, are put off to a late 

 embryonic stage, or, more frequently, till post-embryonic life. The 

 Gnathobdellidae follow the same laws as the Rhynchobdellidse. 



The author makes some critical remarks on Mr. Whitman's explana- 

 tion of the external morphology of the Hirudinea. 



If we sum up what is known as to the external morphology of these 

 forms, we may say that the Hirudinea have an elongated body which, as 

 a rule, tapers at both ends ; it is smooth externally and is provided with 

 regularly distributed folds of the integument; it is always distinctly 

 ringed, and is, in cross section, rounded or horizontally oval ; it only 

 becomes flattened secondarily, and its length is, proximately, due to the 

 number of rings found in each somite. The body is always made up of 

 thirty-three distinct somites, each of which possesses a ganglion consist- 

 ing of six ganglionic capsules, which become more closely approximated 

 to one another at either end than elsewhere, without undergoing any 

 greater modifications. The somites shorten regularly towards either 

 end of the body, and at the same time become more or less reduced. 

 The number of non-reduced somites is characteristic of the genus, but 

 the extent and mode of reduction are generally specific characters only ; 

 they have no direct connection with the phylogeny of the whole order. 



The whole body may be divided into six functionally different 

 regions, all of which, with the exception of the anal, consist of six 

 somites each; they are the cephalic or suctorial region, the clitellar 

 region, the region of the fore- and that of the hind-gut, the anal region, 

 and that of the attaching disc. The cephalic region has, in the interest 

 of a semi-parasitic mode of life, been more or less converted into a 

 sucker; this is a thickening of the end of the body; the apparently 

 secondary anus is in the form of a transverse cleft which, at a late period, 

 breaks through the skin in the middle line of the back. The final 

 region has a disc, the size and form of which is in direct connection 

 with the grade of parasitism, and which, in non-parasitic forms, is chiefly 

 used as a locomotor organ. The clitellum occupies somites 10-12, 

 and is more or less secondarily modified, its form varying in different 

 families ; the male generative orifice is on the eleventh and the female 

 on the twelfth somite. The relative size of the mid-body is an adapta- 

 tion to the quantity of food which the genus has to obtain ; its median 

 segments, or somites 14-23, are typical of genus and species. 



In all Hirudinea there is a highly developed tactile sense, and 

 eighteen longitudinal rows of tactile goblets; in some genera the 

 marginal line is distinguished by larger goblets, and so presents a 

 distinct homology to the lateral lines of the Capitellidse. Eyes, which 

 in the more highly developed genera perceive light, colour, and probably 

 even form, are best developed in the fresh-water forms. 



Specific glands are present in the form of special chitin-glands which 

 are used to form the cocoon, and, when no cocoon is made, as in some 

 species of Clepsine, they form an embryonic attaching gland. 



The author believes that the Hirudinea form an order of Annulata 

 parallel to the Chaetopoda. In later times there aj)pears to be a tendency 



