DORIS. 



numerous otolithes, and rest upon the cerebroid ganglia immediately behind the attachment 

 of the optic ganglia. 



The skin in most species is tough and coriaceous, and is of a spongy or cellular structure 

 internally: the cloak in all the British species is stiffened with numerous imbedded, calcareous 

 spicula, having a more or less symmetrical arrangement. Spicula are also observable in the 

 foot, in the tentacles, and in the branchiae. The under surface of the foot, and the upper 

 surface of the cloak, as before stated, are covered with vibratile cilia. The branchial plumes 

 and dorsal tentacles are also furnished with them. 



The ova of Boris (PI. 3, fig. 8) are generally arranged in transverse rows, imbedded in a 

 transparent gelatinous belt, attached by one of its edges to some foreign substance, and 

 coiled one or more times into a pretty regular cup- or vase-like form. In one division of our 

 second section, however, the ova are contained in a fine, depressed, close, thread-like coil. 

 The ova are very numerous. On a moderate computation, there cannot be less than 50,000 

 in a single patch of spawn (PL 3, fig. 7) of I). tuber culata ; and when we take into account 

 that all the individuals are prolific — the sexes being combined — and that each deposits two 

 or three patches in a season, it is evident how vast must be the number of young produced 

 each year, a single pair bringing forth probably 300,000 eggs. 



Most of the species deposit their spawn in spring, and shortly afterwards the yolk, which 

 is contained within a delicate, transparent, membranous shell or chorion, may be observed 

 changing form (PI. 3, figs. 9, 10, 11, 12). The time required to mature the embryo varies in 

 different species, and probably in the same species, under different circumstances. The late 

 Dr. John Reid* found that the spawn of D. bilameUata was hatched in fourteen days, and a patch 

 of the spawn of D. tuberculata, removed by us from the rocks very soon after deposition, came 

 to maturity in fifteen days. The whole of the ova in the same mass of spawn, however, are 

 not hatched simultaneously, their successive liberation sometimes extending over a period of 

 several days. As the character of the embryo and mode of development are extremely similar 

 throughout the Nudibranchiate Mollusca, to avoid repetition, we purpose extending this 

 account of the embryology to the whole order, and may therefore here state that we have 

 ascertained that the spawn of Polycera quadrilineata is hatched in ten or twelve days ; that 

 of Doto coronata in eight or ten, of Hermcea dendritica in nine, and of Eolis punctata in ten or 

 eleven. In all these cases the spawn was kept in the house, and probably the consequent 

 increased temperature may have hurried the development. Professor Nordmannf found the 

 development of his Tergipes JEdwardsii to require from sixteen to twenty days. 



In Doris tuberculata the chorion is elliptical, and generally contains one yolk, sometimes 

 two or three : at first the yolk j is round, and nearly fills the chorion (PI. 3, fig. 9) ; it soon 

 becomes a little elongated, with one end diagonally truncated (fig. 10) ; the truncated 

 end then becomes bilobed, each lobe exhibiting an imperfect spiral, and having its 

 margin ciliated (figs. 11, 12). The now animated being is seen to rotate within its prison. 

 Shortly, the lobes, which are placed in front, enlarge, and a fleshy process — the rudimentary 



* Ann. Nat. Hist., v. 17, p. 388. 

 t Ann. des. Scien. Nat., 3d ser., v. 5, p. 144. 



X The progressive cleaving or breaking up of the yolk has been minutely described by Dr. Reid. 

 Loc. cit. 



