Mr. T. H. Stewart on the Anatomy of the Echinoderms. 71 



auriculae of Cidaris do not form au arch, as they do in Echinus, Dia- 

 dema, and other genera. 



The oral skeleton of Clypeaster is a modification of that of 

 Echinus, but more simple. The Ophiuridce have also a decided oral 

 apparatus, differentiated from the other calcareous portions of their 

 skeleton ; it is a decidedly modified form of the splanchnic skeleton 

 of Echinus. They have alveoli somewhat like those of Ctypeaster ; 

 a perforated oblong plate is situated perpendicularly at the symphyses 

 of these alveoli ; and here are situated a number of small, square, 

 chisel-shaped teeth, translucent and sharp at their free end, and thick 

 and opake at the attached end or root : there are five of these on 

 each of the plates, and their points can meet so as to close the oral 

 orifice. The Goniasters and the Asterince also, by means of their 

 oral spines, can completely prevent the egress of food once taken 

 into the stomachal cavity. The Solasters have beautiful fasciculated 

 spines around the mouth ; but they only partially or very slightly 

 close the orifice, which in this Starfish is very capacious. The 

 Urasters have five bundles, not quite closing the mouth. 



There are some other points to be mentioned with regard to the 

 antambulacral or anal ring of plates in the Echinidce. This ring 

 of plates is all in the Echinus that answers to the upper or ant- 

 ambulacral integument in the Star-fish. 



The number of plates forming this part of the corona is ten, — five 

 situated interambulacrally, i. e. one at the anal end of each inter- 

 ambulacrum ; and five ambulacrally, or one at the end of each am- 

 bulacrum. The first are known as the genital plates ; they are some- 

 what triangular in shape, with a semicircular border towards the 

 peripygial membrane ; these plates have a perforation for the exit 

 of the generative products. Intermediately between these genital 

 plates in the anal ring and at the anal end of the ambulacral series 

 are much smaller triangular plates : these are called "ocular plates;" 

 but as the Echinus has no eye, it is erroneous so to call them, and 

 therefore I propose the name of "inter- genital" for them. At the 

 point furthest from the anus they have a minute perforation ; now 

 to this perforation can be very readily traced the longitudinal ambu- 

 lacral vessels that are placed on a raised edge in the centre of each 

 ambulacrum, and terminate at this orifice in the inter-genital plates, 

 which, no doubt, is the orifice of exit of the fluid used in the ambu- 

 latory system after it has circulated in the body and done its service 

 and is no longer of any use. The anus in Echinus is excentric ; in 

 Cidaris it is centric. 



In connexion with the position of the Echinoidea in the animal 

 series, it is interesting to know that, as in the class of polyps below 

 them, there are spicula scattered loosely through their tissues, as well 

 as agglomerated ones forming their corona. I have found spicula in 

 the fleshy tubes of the cirri, in the membranous madreporic canal, 

 in the generative organs, in the fleshy part of the stem of the pedi- 

 cellaria, and, very curiously, along the border of a spine that I have 

 mounted in longitudinal section for microscopic observation. These 

 spicula are much like those in some Sponges, except that in Echinus 



