ECHINOIDEA. 561 



sticha. The apparatus is moved by special muscles of which some are 

 connected to the auricles. The Palaeo-echinoidea possessed similar masti- 

 catory organs. There is an oesophagus, followed by an intestine ending in 

 a rectum. The commencement of the intestine is marked by a small 

 dilatation in Desmosticha, by a large caecum in Petalosticha. The oeso- 

 phagus lies in the madreporic interradius : the intestine is arranged in two 

 coils, an inferior and a superior. The former passes through radius III to 

 IV, V, I and II (see Loven's formula, note, p. 555) into the madreporic 

 interradius, where the latter commences and turns back through radius II, 

 to I, V and IV. The rectum commences in the interradius between III 

 and IV, and runs vertically or obliquely to the anus. Mesenteric bands 

 connect the coils to the test, and in Petalosticha to one another. In the 

 Desmosticha and Petalosticha a tube the siphon arises from the posterior 

 extremity of the oesophagus and lies closely applied to the inner margin of 

 the intestine into which it opens again at or near the end of the inferior 

 coil. It is the 'convoluted organ' so-called of Spatangus. A second 

 siphon has been observed in some Petalosticha (Schizaster, Brissus^ &c.). 

 There is a special anal muscle. The position of the anus has been already 

 pointed out. The pedicellariae have been observed removing the faeces 

 from the apex of the shell. 



The generative glands are arborescent caeca, large when sexually 

 mature in Desmosticha^ opening by a single duct on the basals or in the 

 interambulacra (viviparous Cidaridae ; some Ctypeastroided). They are 

 typically five, but, as already stated, the number is often reduced. The 

 ripe testes are milk-white, the ovaries yellowish-brown. 



The free swimming larva is a Pluteus. It differs from the Ophiuroid 

 Pluteus in the absence of lateral arms, and the presence of an antero- 

 internal pair to which are added in some Petalosticha an antero-external 

 pair and three processes from the post-anal area. Many Echinid Plutei 

 have a pair of ciliated ' epaulettes ' on each side behind the ciliated band 

 from which they are developed originally. They are not present in the 

 Pluteus of Dorocidaris. 



The deep-sea Urchins are related to forms prevalent in the Chalk. 

 The Palaeo-echinoidea appear in the Lower Silurian strata. The oldest 

 exocyclic forms are found in the Lias. Typical Clypeastroidea occur in the 

 Upper Chalk, small however in size, and the larger genera develope from 

 the Miocene to the present time. Typical Spatangideae appear in the 

 Lower Chalk ; the older forms (Collyrites, Clypeiis] in middle Jurassic 

 strata. 



The Echinoidea may be classified into 



i. Palaeo-echinoidea\ with a variable number of meridional rows of inter- 

 ambulacral and sometimes of ambulacral plates. The plates overlap one another, 



o o 



