208 LECTURE X. 



they proceed, supplying the internal branchice and the ambulacral 

 tube-feet ; they finally terminate by penetrating the pore of the ocular 

 plate to gain the base of the red ocellus. 



The generative apparatus of the Echinus consists of five mem- 

 branous sacs {jiy. 98. h, h), the efferent ducts of Avhich perforate 

 five plates, sui'rounding the anal plates, and thence called genital or 

 ovarian plates. This structure is common to both sexes, Avhich are 

 in distinct individuals in the Echinoids, as in the Star-fishes. The 

 ovaria, when distended, with the mature ova, which generally present 

 a bright oi'ange colour, fill a great part of the cavity of the shell, and 

 resemble the ovaria or roe of fishes. They have at all periods con- 

 stituted a favourite article of food with the inhabitants of the 

 Mediterranean shores. 



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

 germinal vesicle, and its solid 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 ex- 

 emplified by the frequent repetition of similar parts in the multiplied 

 mouths of the Polypi, the multiplied stomachs of the Polygastria, the 

 multiplied ovaria in the Taeniae, and the multiplied marsupia in the 

 Medusae. If we view the articulated moveable spines and the ex- 

 tensile 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. 



Holothurioidea. The Holothuria, which is the highest organised 

 of the Echinoderms, may be compai-ed, as has been already observed, 

 to an Echinus deprived of its spines, wath its shell softened and 

 elongated by divarication of its poles. The coriaceous integument is 

 strengthened by scattered areolar calcareous concretions, and con- 

 tinues to be perforated by innumerable apertures, which give passage 

 to tubular feet of precisely the same structure as those in the sea- 

 urchins and star-fishes. These tube-feet are likewise in some species 

 of HolothuricB disposed in five longitudinal ambulacral series ; in a 

 few species {Psoitis Oken) they are confined to a sort of ventral 

 disc: in other species they are generally difi"used over the integu- 

 ment. The chief calcareous substances in this coriaceous integument 

 consist of a circle of osseous pieces, usually ten in number, and 

 homologous with the parts of the "lantern" in Echinus, which 



