ECHINODERMA. 125 



rounding the anal plates, and thence called genital or ovarian plates. 

 This structure is common to both sexes, which are in distinct indi- 

 viduals in the Echini, as in the Star-fishes. The ovaria, when dis- 

 tended with the mature ova, which generally present a bright orange 

 colour, fill a great part of the cavity of the shell, and resemble the 

 ovaria or roe of fishes. They have at all periods constituted a 

 favourite article of food with the inhabitants of the Mediterranean 

 shores. 



The ova consist of a vitelline membrane, vitellus, the transparent 

 germinal vesicle, and its simple nucleus. 



The spermatic corpuscles are elongated, oval, rounded anteriorly, 

 pointed behind. They abound in the opake milky fluid, distending 

 the five secerning sacculi at the breeding season. 



In the multiplicity of the pieces of which the shell of the Echinus 

 is formed, we may discern, by^the contrast which it presents with the 

 bivalve and univalve characters of the shells of the Mollusca, the same 

 low vegetative condition of an external skeleton which is exemplified 

 by the frequent repetition of similar parts in the multiplied mouths of 

 the Polypi, the multiplied stomachs of the Polygastria, and the mul- 

 tiplied ovaria in the Taeniae. If we view the articulated moveable 

 spines and the extensile and prehensile tubes in the light of primitive 

 forms of locomotive extremities, we shall see in their great numbers 

 and irrelative repetition, an illustration of the same law. 



HolothuriidcB. The HolotJmria, the highest of the EcMnodermay 

 may be compared, as has been already observed, to an Echinus de- 

 prived of its spines, with its shell softened and elongated by diva- 

 rication of its poles. The coriaceous integument continues to be 

 perforated by innumerable apertures^ which give passage to tubular 

 i^iiX, of precisely the same structure as those in the sea-urchins and 

 star-fishes. These tentacles are likewise in some species of Holothurice 

 disposed in five longitudinal ambulacral series ; in a few species 

 (Psolus Oken) they are confined to a sort of ventral disc : in other 

 species the suckers are generally diffused over the integument. The 

 only calcareous substances in this coriaceous integument consist of 

 a circle of osseous pieces, which partly defend the nervous ring, and 

 which afford a firm attachment to the branched retractile tentacles 

 which surround the mouth. These tentacula may be likened to a 

 more complicated form of the ordinary tubuli of the body, each 

 being connected at its base with a long hollow sacculus, and being 

 distended and protruded by the injection of the fluid contained in that 

 sacculus. 



The alimentary canal is closely analogous to that of the Echinus ; 

 but its disposition is accommodated to the vermiform character of 



