ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 801 



and B. robinii than in B. Kowalevskii ; their peripheral ends are very 

 long and fine, and occasionally branch, while their central ends taper 

 suddenly ; the former are probably inserted into the skin, and the 

 latter into the meshes of connective tissue which permeate the body- 

 cavity. 



Passing from the outside to the centre of the proboscis, the parts 

 are found to be thus arranged : — 



1. Ectoderm. — Ciliated tailed cells. 



Glandular cells. 

 Nerve-fibres as a layer. 

 Basement membrane. 



2. Narrow tissue space crossed by fibres from the ectoderm, by 

 supporting fibres going in all directions, and a very few circular fibres. 



3. Tract densely filled with radial and longitudinal muscles and 

 connective tissue. 



4. The tissue space into which the central organs project. 



5. The central organs : — 



(a) Proboscis gland, with its sac. 

 (&) Heart, 

 (c) Notochord. 



In B. Kowalevskii and B. salmoneus the dorsal mesentery persists 

 throughout life ; in other species it disappears in the region of the 

 collar; the ventral mesentery persists in the trunk, but is always 

 obliterated in the collar. 



The proboscis gland first appears as a space in the proliferation 

 of mesoblast lying dorsally to the anterior end of the notochord, at 

 about the age of two gill-slits ; it soon becomes inclosed in a mem- 

 brane, and its cavity commxmicates with the body-cavity by means of 

 the intestines between the cells bounding its anterior end. The heart 

 arises at the age of three gill-slits, as a horizontal split in the tissue 

 between the notochord and the sac of the proboscis gland ; its walls 

 are very thin, and soon become slightly muscular. 



There is evidence in support of the view that the ovaries are of 

 epiblastic origin, and they are at any rate from almost their earliest 

 appearance connected with the skin in the dorso-lateral regions ; they 

 at first consist of a mass of loose round cells. The testes occupy 

 the same position as the ovaries, and form lobed masses ; the outer 

 zone is made up of spherical cells which contain several dots ; the 

 cells are young spermatoblasts, and the dots the heads of spermatozoa. 



Echinodermata. 



Circulatory System of Echinoids.* — M. E. Koehler has come 

 to the conclusion that M. Prouho is right in asserting the exist- 

 ence of five pharyngeal vessels in Dorocidaris, and he has been able 

 to extend this observation to other genera of Echinoids. He does 

 not, however, agree with that naturalist in his view of the homology 

 of the sand-canal of Spatangus with the Polian or peri-cesophageal 

 rings of the Cidaridae ; he thinks that the latter are rather homologous 



* Comptes Rendus, ciii. (1886) pp. 86-8. 

 Ser. 2.— Vol. VI. 3 G 



