THE SEA-URCHINS. 271 



mentioned, which are well shown in Fig. 15, A. Two other sets of accessory pieces connect the 

 pyramids together, and serve as attachments for muscles, the number of these organs which are 

 concerned in moving the whole lantern being thirty-five. 



The teeth move concentrically around the opening of the gullet (Fig. 16, ce), which passes 

 upwards through the lantern, and is continued into an elongated digestive tube (Fig. 16, t). 

 This exhibits no differentiation into stomach and intestine, but is coiled spirally around the 

 interior of the test, to which it is attached by a mesentery. It is accompanied by two blood- 

 vessels the one dorsal and the other ventral, which are connected with one another by an 

 extensive vascular network in its walls. The i, t -, 



ventral vessel arises from an oral ring, which po 

 is situated, together with the water-vascular 

 ring, on the upper surface of the lantern. 

 It is probably (though we do not as yet know 

 with certainty) connected with an aboral ring, 

 from which the vessels supplying the genital 

 glands are given off, and in which the dorsal 

 intestinal vessel may perhaps arise. The cen- 

 tral plexus is in intimate relation with the 

 water-tube which descends from the madreporite Fig i 6 ._ V i E w OF THE INTERIOR OF THE BISECTED TEST OF 

 to the water-vascular ring. This last usually THE PURPLE EGG-URCHIN (Echinus liwdws). 



U~, fl,^, T>^li nl ^l^c, nrl rriVoc. rvflf* +Vio ">' fc' 11 "^ ' '. intestine ; a, amis ; p, a water-vessel ; po, oral end of another ; ca. 



bears live .r Olian VeSlCleS, and gives On tne ocular plate ; , one of tho pieces of the dental pyramid ; m, one of the jaw 

 ,.-- 1,1 -i f j_i muscles; v, ovary. (After Tledeminn.) 



radial vessels, which descend the sides ot the 



lantern, and then pass outwards beneath the arches of the auricles (Fig. 1 6, p, po). The bases of 

 the lateral tentacular branches which they give off open into large ambulacral vesicles, just as in the 

 Stellerids. These radial water-vessels are accompanied by the radial blood-vessels and nervous trunks. 

 The latter start from an oral ring, which is not above the lantern as the vascular rings are, but is close 

 down upon the buccal membrane lying between the gullet and the tips of the teeth, which project 

 from the lantern. The tentacular branches of the radial, nerves pass outwards through the same 

 pores in the ambulacral plates as the tentacles themselves, and also communicate with an extensive 

 nervous network, which penetrates the delicate membranous layer surrounding the test, and furnishes 

 nerves to the pedicellarise and spines. 



In most of the regular Urchins there are ten gills in the neighbourhood of the mouth. These 

 are thin-walled ciliated extensions of the closed body-cavity, which protrude between the buccal 

 membrane and the lowest plates of the test, and assist in the work of respiration. In the irregular 

 Urchins this function is exclusively performed by the water- vascular system, and some of the 

 tentacles are specially modified, becoming broad, flat, and somewhat lobed. These are often 

 spoken of as ambulacral gills. 



The genital glands of the Urchins are situated in clusters beneath the aboral portion of 

 the test, and communicate with the exterior by the pores in the genital plates. 



All the Urchins are gregarious, and many of the Desmosticha inhabiting coasts that are 

 much exposed to the action of the waves protect themselves by hollowing out cavities in the 

 solid rock, even in granite. This is the case with the purple Egg-urchins of the English coast. 

 They chisel out the rock with their teeth by incessantly turning round and round, commencing 

 when young, and continually enlarging their prison to allow for the growth of their test and 

 spines. The irregular Urchins, on the other hand, mostly prefer quiet sandy places, where they 

 can bury themselves. 



(4) HolothuToidea. The Holothurians, which are also known as Sea-cucumbers, Trepangs, or 

 Beches de Mer, are the most worm-like and the least radiate in form of all the Echinoderms. 

 They have more or less elongated bodies (Fig. 17) enclosed in a tough skin, which contains only 

 a comparatively small amount of calcareous matter ; and this (except in rare cases) never forms 

 a continuous armour of plates, but occurs only in the shape of scattered grains, which often 

 assume very definite and regular forms. There may, however, be a ring of limestone plates 

 around the gullet, five of which have the same relation to the radial water-vessels as the auricles 



