220 LECTURE X. 



the abdominal cavity. The males show in the same fluid a like 

 abundance of subdepressed roundish heaps of aggregated vesicles or 

 cells full of gi'anular matter. These are the formative cells of the 

 spermatozoa : they continue in this undeveloped state all the winter, 

 but in the early spring the surface of the granular masses begins 

 to be beset with minute filaments, and in tlie middle of April the 

 liberated spermatozoa abound in the chylaqueous fluid. 



The chorion of the mature ova is uneven, and its surface is beset 

 with oblong nucleated cells : the yolk is inclosed by a firm mem- 

 brana vitelli, with a facetted surface, like the cornea of insects : it is 

 divided by some limpid fluid from the chorion. The yolk is minutely 

 granular, and contains a clear germinal vesicle with a solid nucleus. 

 The embryo, when developed, carries out with it part of the peculiarly 

 facetted vitelline membrane, of which it afterwards disembarrasses 

 itself. The broad semicircular head of the larva supports a pair of 

 pigment- or eye-specks, behind which it is encircled by a ciliated band. 

 The alimentary canal floats in an abdominal cavity : the pharynx is 

 divided by a constriction from the capacious stomach : a short and 

 narrow intestine is twice bent, its last fold returning to terminate on 

 the dorsal surface of the body, and thus early indicating the charac- 

 teristic position of the vent in the adult. The two blind sacs are 

 short and wide, and open on the sides of the ventral surface of the 

 abdomen. There is a pair of small spherical glands beneath the 

 pharynx, with a common duct which opens just in advance of the 

 ciliated band. A pair of retractor muscles proceed from both the 

 dorsal and ventral aspects, to be inserted into the band for its re- 

 traction, and five or six more feebly developed transverse bands for 

 the constriction and elongation of the body may be discerned.* 



Thus it appears that, diversified as are the mature forms of the 

 Echinodermata, each begins its free and locomotive existence as an 

 oval ciliated larva, like a polygastric animalcule : the variations in 

 the subsequent metamorphoses are thus summed up by Professor 

 Miiller: — 



*' 1. — The change of the bilateral larva into the Echinoderm takes 

 place when the larva yet remains an embryo, and is wholly covered 

 by cilia, without a ciliated fringe. A part of the body of the larva 

 takes on the form of the echinoderm : the rest is absorbed by the 

 latter. (A part of the Stclleridce, e. g., Echinaster, Asteracanthion.) 



" 2. — The change of the bilateral larva into the Echinoderm takes 

 place when the larva is perfectly organised ; that is, possesses 

 digestive organs and a ciliated fringe. The Echinoderm is con- 



* CLXVIII. 



