31 



stone, -which so much resembles that of wood, as to be by some mis- 

 taken for a petrifaction. They receive a most exquisite polish, the 

 edges are very curious and the tops beautifully clouded. Dr. 

 Peacock called them the flowered jasper, and others Egyptian, the 

 Sicilian, and the Antique Alabaster. The large specimen I place 

 before you, all will admit is worth some consideration. 



MAT. 



Oil the Development of the Doris Tulerculata, ly Mr. Sihert Saunders. 



Mr. Saunders exhibited some microscopic marine zoophytes and 

 polyzoa, sertula pumila, Bower baukia, Alcyonidium, gelatinosum, 

 and Bugula avicularis, and gave the following description of the 

 development of Doris tuberculata, the eggs of which species he also 

 exhibited. — The eggs of the Nudibranchiate mollusks, of which 

 Doris tuberculata is one of the largest and commonest species, are 

 (like those of Lymnceus stagualis and other fresh water snails) de- 

 posited in a mass enveloped in a jelly-like substance ; but, instead 

 of the 50 or 60 eggs deposited at one time by Lymnoeus, the naked- 

 gilled mollusk will lay many thousands, and, in the course of the 

 spawning season, will produce several hundred thousands of eggs. 

 The substance in which these eggs are deposited also differs from that 

 of the fresh water snails, being much more delicate both in colour 

 and texture, and its perfect transparency makes it a suitable and 

 interesting subject for the microscopist. In the example now under 

 examination, the gelatinous envelope is in the form of a long ribbon 

 of tolerably uniform width, attached by one edge to a piece of sea 

 weed, and frilled or coiled spirally. "When first deposited the jelly- 

 like substance of this ribbon is filled with minute spherical bodies, 

 each of which is in due time divided and sub divided, until a mass 

 of yolk segments is formed within each ovum. It now becomes very 

 interesting to watch the motion of the eggs, which are seen to re- 

 volve several times in one direction, and then several times in the 

 opposite direction. This motion is caused by cilia which are now 

 being developed on the anterior or cephalic portion of the embryonic 

 mass. This portion grows rapidly, and in a short time the foot or 

 muscular disk of the animal is formed. A shell is also formed in 

 the posterior portion, into which, as soon as it is large enough, the 

 embryo contracts itself. It is remarkable that the shell is thus 

 formed during the embryonic stage of all the gasteropods, although 

 in the case of the nudibranchs it is cast off' after they leave the egg, 

 and in some other families it continues to exist only as a ihin cal- 

 careous plate imbedded in the substance of their muscular covering. 

 The head of the embryo Doris becomes furni.'shed with two mem- 

 branous lobes, richly ciliated, and wh'n tlie time arri\ es for its ex- 

 trusion the egg case is ruptured and the young mollusk swims away 



