HIRUDINEA. 519 



from the longitudinal canals of the sinus system by having thicker, 

 often more muscular walls. The main longitudinal trunks vary 

 considerably in different genera (Fig. 420), but however many there 

 are they all communicate with one another and form a continuous 

 system, and are often contained in one or more of the canals of the 

 coelomic system. 



In Pontobdella there are four main trunks, a dorsal and ventral and two lateral, 

 all contained in sinuses ; in Clepsine there is a dorsal and a ventral, both con- 

 tained in sinuses ; in Hirudo and Nephelis there are no dorsal and ventral vessels, 

 but only two lateral, which are not contained in sinuses. 



In Clepsine the dorsal vessel is dilated into fifteen small chambers, each of 

 which possesses a cellular projection of its wall, which is called a valve and 

 probably buds off blood corpuscles. Moreover, the hinder part of the dorsal 

 vessel in this animal spreads out into a sinus, which completely surrounds the 

 intestine and opens into the ventral vessel. 



The botryoidal tissue is found in the GnathoMelUdae. It is a 

 brown pigmented tissue which embraces the alimentary canal, the 

 blood vessels, the main lacunae of the coelom, etc., "in a word, it is 

 a pigmented tissue, which in the absence of a proper perivisceral 

 cavity fills up all the interstices situated between the organs it 

 invests " (Leydig quoted by Lankester). It consists of swollen cells 

 joined together in anastomosing rows, and containing tubular cavities 

 continuous along the cells of a row. These botryoidal vessels contain 

 a red fluid, and constitute a tubular network lying between the 

 alimentary canal and body-wall, and extending into the muscles of 

 the latter. In places they are swollen into the botryoidal sinuses, 

 which are vesicles lined by botryoidal cells. These are conspicuously 

 developed in Nephelis and Trochetia (forms without the dorsal 

 sinus), and some of them contain and receive the opening of the 

 funnels of the nephridia. They are comparable to the so-called 

 perinephrostomial sinuses, or parts of the sinus system which contain 

 the nephridial funnels in other Gnathobdellidae. 



In Hirudo a similar, but less conspicuous structure, formed of botryoidal 

 tissue and containing the funnel of the nephridium, is present on the dorsal side 

 of the testis. 



The botryoidal vessels undoubtedly communicate with the dorsal and ventral 

 sinuses, or with extensions of these. From their development (see below) there 

 can be no doubt of their coelomic nature, and they clearly correspond to the 

 network of tubes which connect together the main sinuses of the Rhyncho- 

 bdellidae, in which group botryoidal tissue is absent. As in this group they 

 contain a fluid similar in colour and character to the blood (red in the Gnatho- 

 bdellidae), and this similarity has led to the view that the sinus system and 

 vascular system are continuous in both divisions of the Hirudinea. As we have 

 already seen (p. 517), no observations have been brought in support of this for the 



