ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 213 



been claimed, and in every case finds pores absent, or in such position 

 that it seems they are either connected with the functional byssogenous 

 organ, or, where such is absent in the adult, with the remnant of the 

 same. Barrois sums up his views thus : — No pores exist for the in- 

 troduction of water into the circulation ; the only pores of the foot 

 are those connected with the byssus organ, which never communicates 

 with the interior of the foot. The blood may have water introduced 

 into it, but this may be effected by osmosis, or in some manner not 

 discussed. 



Visual Organs in Solen.* — B. Sharp, referring to his recent 

 communication f on the visual organs of Solen ensis, states that he 

 had since determined the presence of similar organs in the mantles 

 of the clam, the oyster, and the sand-clam. Their presence was 

 made evident by the retraction of the mantles when shadows are 

 passed over them. The structure of the peculiar cells supposed to 

 be primitive eyes, was the same as that of the cells before described 

 in the siphon of Solen, including the presence of the transparent 

 portion at the end of each. 



Molluscoida. 



"Egg and Egg-membranes of Tunicata. | — H. Fol finds that the 

 mature ovum of a Tunicate is composed of a granular yolk, containing 

 a female pronucleus, with two polar globules at the surface. A 

 gelatinous layer, containing a very large number of non-nucleated 

 corpuscles, surrounds both the yolk and the globules, while the 

 surface of the whole is occupied by a layer of nucleated cells, which 

 forms the follicle ; sometimes this envelope is double, when the outer 

 layer is composed of flattened cells united to form a continuous mem- 

 brane. The polar globules and the pronucleus are derived from the 

 germinal vesicle, in which a nucleolus, a nuclear plexus, and an envelope 

 could be detected. The corpuscles of the larval testa are homogeneous, 

 and inclose a certain number of large yellowish granulations ; at first 

 they are of a vesicular character, but at no time have they a true 

 nucleus ; they owe their origin to the superficial portion of the yolk, 

 and arise from it about the time when the egg has attained to half 

 its permanent size. In a Molgula these corpuscles were found to be 

 replaced by nucleated cells, which were distended by the homogeneous 

 masses which they contained. The overlying nucleated cells are, in 

 a majority of cases, largely vacuolated, but this appearance is not to 

 be made out before the layer is complete and the cells have been for 

 some time superficial in position ; their nucleus arises as a small 

 solid or hollow bud from the germinal vesicle, while the body of the 

 cell is derived from the yolk : when these cells are long in appearing 

 the germinal vesicle is apparent for the whole period ; when, on the 

 other hand, they arise rapidly and in large numbers simultaneously, 



* Ptoc. Acad. Nat. Sci. PMlad., 1884. See Science, iii, (1884) p. 237. 



t See this Journal, ante, p. 39. 



X Recueil Zool. Suisse, i. (1883) pp. 91-160 (2 pis.). 



