TYPE ECHINODERMA. 559 



they are surrounded and may be covered in by the movable 

 spines of the adambiilacral and marginal plates. That they 

 have a sensory function seems clear, but what the exact 

 nature of the function may be is as yet uncertain. At the 

 base of the terminal tentacle of each arm is situated an eye, 

 consisting of a large number of conical depressions, lined by 

 an epithelium containiug a red pigment, covered on the out- 

 side by a cuticle and richly supplied with nerve-filrils. There 

 do not seem to be present any refractive structures other than 

 the cuticle, and these eyes can onl}- convey to the animal im- 

 pressions of changes in the intensity of the light falling upon 

 them ; they cannot form images of external objects. 



The reproductive organs are ten in number, two being 

 situated in each arm (Fig. 255, g). Each consists of a mass 

 of reproductive cells, and is enclosed in a genital sinus (Fig. 

 •254, I), which, as already stated, communicates with the axial 

 sinus. The proximal end of each gland is connected with a 

 cordlike structure, the genital cord or rachis, the ten cords 

 uniting in a ring situated beneath the aboral surface of the 

 body, a cord passing orally from this ring to unite with the 

 tissue of the ovoid gland. The genital sinuses accompany the 

 cords, enclose the ring, and pass to the axial sinus along with 

 the descending cord. A connection therefore exists between 

 the reproductive organs and the ovoid gland, just as in the 

 Crinoids, and indeed the reproductive organs arise in the 

 embryo from an outgrowth of the ovoid gland. In reality the 

 genital cords are tubes containing in their interior immature 

 reproductive cells which seem to migrate from the ovoid gland 

 to the reproductive organs where they become mature. The 

 reproductive openings are usually placed upon the aboral 

 surface {Asterina forming an exception) of the arms or disk in 

 the interradii ; a single pore usually exists for each gland, 

 and occasionally there may be several. 



Development of the Asteroidea. — The larval forms of the Starfishes are 

 known as the Bipinnaria and Brachiolaria. The former has a somewhat 

 triangular shape, the apex of the triangle being the anterior extremity, 

 and in the middle of the ventral surface is a deep concavity in which the 

 mouth opens. The posterior border of the concavity is formed by a band 

 of cilia which is continued around the lobed sides of the body to the an- 



