ix . PHYLUM ECHINODERMATA 409 



ring (ect. ne) surrounds the mouth, and from it are given off 

 a series of nerves — thickenings of the epidermis of the ambulacral 

 grooves and their offsets — which extend throughout the length 

 of the arms and pinnules. The deep nervous system follows the 

 same general arrangement as the superficial. In the axis of the 

 supporting ossicles of the arm is an axial nerve (ax. co), which gives 

 off branches (Fig. 333, ax. ne) running through the axes of the 

 ossicles of the pinnules. The axial nerves are connected internally, 

 not with the circum-oral nerve-ring, but with a central body 

 situated below the rosette, in the interior of the centro-dorsal ossicle. 

 This, the central capsule (Fig. 334, cent, caps), forms the investment 

 of a body termed the five-chambered organ (chamb. org), divided 

 into five parts by radial septa, and continuous with the aboral end 

 of the genital stolon. Processes from the five angles of the central 

 capsule combine to form a pentagonal ring from which pass out- 

 wards the axial nerves of the arms. Aborally the central capsule 

 gives off nerves to the cirri. 



A system corresponding to the perihaemal system of the 

 Starfish is present, though reduced, and there is a highly developed 

 and complicated lacunar or haemal system. 



Numerous bodies termed the sacculi, the character of which 

 has given rise to much discussion, occur regularly arranged along 

 the ambulacral grooves and also in other parts. They are small, . 

 spherical bodies which become vividly coloured when treated with 

 staining agents. They are sometimes supposed to be parasitic 

 Algae ; but the regularity of their arrangement is opposed to 

 such a view. • It has been suggested with more appearance of 

 probability that they may be masses of reserve materials, stored 

 up for the nutrition of the animal, or may consist of excretory 

 matters. 



The reproductive organs — ovaries or testes, as the case may 

 be — are lodged in the dilated bases of the pinnules, which become 

 considerably enlarged as the ova or sperms mature, those next to 

 the bases of the arms alone remaining sterile. When mature, the 

 sexual elements escape by means of short ducts. Each gonad 

 is one of the terminal parts of a system of tubes lined by an 

 epithelium, and extending from a central part or genital stolon 

 {gen. st) — lodged in the vascular plexus that surrounds the oesopha- 

 gus and connected dorsally with the chambered organ — outwards 

 through the arms ; the terminal portions, lying in the pinnules, 

 aie dilated to form the reproductive organs, and the cells 

 of their epithelium become developed into ova or sperms, while 

 the rest constitute a non-fertile connecting rachis. This system 

 is enclosed throughout by a plexus of ha?mal lacunae. 



Like the rest of the Echinoderms, the Feather-star undergoes a 

 metamorphosis ( Fjgs. 344 and 345). It passes through a free- 

 swimming ciliated larval stage, which is followed by a fixed 



