VELELLA. in 



The proximal wall of the hydrosoma, or that which covers the upper surface of the disc, 

 presents a system of similarly ciliated, wide, anastomosing canals, which communicate with a 

 longitudinal canal, situated, over the diagonal groove which marks the upper surface of the 

 liorizontal portion of the pncumatocystJ On the limb of the disc these canals divide 

 dicliotomously into straight sacculated branches, which end in a marginal canal. 



All these ciliated sinuses are continuous with a series of similar sinuses in the distal 

 layer of the hydrosoma investing the under surface of the animal, which eventually commu- 

 nicate with the cavities of the great central polypite and of the small or gonoblastidial polypites. 



The roof of the wide digestive cavity of the central polypite is, in fact, formed by a dark- 

 brown mass, which fills the inferior hollow of the pneumatocyst, and may probably be regarded 

 as a liver, while, according to KoUiker, it is traversed in all directions by thin-walled canals or 

 sinuses, jigth — ^th of an inch in diameter, containing cells with brownish granular contents. 

 The canals commence in a series of cleft-like apertures, discovered by Krohn, which are readily 

 visible with the naked eye, in the roof of the cavity of the polypite, and from these, forming 

 many anastomoses, the sinuses pass towards the convex surface of the organ. Here they 

 communicate with a whitish network of superficial canals, containing cells, like those of the 

 liver, but colourless. 



At the margins of the liver these whitish vessels become continuous with the system of 

 dorsal canals above described, and, according to KoUikcr, in the following way. Very 

 numerous and close-set ofi'shoots of the whitish vessels pass from the margin of the liver on 

 all sides, into that portion of the soft substance of the body which covers the under surface 

 of the horizontal division of the pncumatocj^st, and, frequently dividing and anastomosing, 

 pass beyond the points of attachment of the gonoblastidial polypites and tentacles to the margin 

 of the pneumatocyst. Here they divide into two sets. Some pass into the limb, and, keeping 

 near its under surface, subdivide and anastomose, becoming continually smaller, until they 

 nearly reach the free edge ; the others, on the other hand, bend upwards, round the edge of the 

 pneumatocyst, and enter those which ramify on its upper surface. 



The sinus which I have described as running along the edge of the vertical plate of the 

 pneumatocyst appears to be what Kolliker describes as the " marginal vessels of the crest " 

 (Randgefasse der senkrechten platte), which he states arise from the ends of the liver, 

 and passing along the line of insertion of the membranous limb, meet and open into one 

 another. 



Besides these ramifications the sinuses of the distal layer of the hydrosoma all commu- 

 nicate with the cavities of the small polypites, the canals of whose peduncles open into them. 



The tentacles (fig. 3) of those VelellcB which 1 examined, appeared to be nearly solid, the 

 endoderm being, as in many Sertulariacia;, so extensively vacuolated, that the sides meet and 

 nearly obliterate the central cavity. Thread-cells^ abound more particularly towards their 

 slightly enlarged ends. Kolliker's account of the structure of the tentacles, however, differs 

 widely from this, since he states that, in V. spiralis, they possess an external longitudinally 



^ According to Kolliker this canal is continued on to the crest in the prolongation of the groove 

 upon the vertical plate noticed above. 



^ These are, according to Kolliker, arranged in two rows in Velella spirans. In a species from the 

 Pacific Leuckart observed four rows. 



