V. Echinoidea. 15 



gradually from below upwards. The apex of a spine is thus its youngest part, 

 while in the pedicellariae the head is the oldest part. A growing spine, when 

 broken, is repaired by the activity of the skin around it; a mature spine, broken 

 near the end, cannot be repaired, but if it is broken near the base, the remnant is 

 thrown off, and a new spine developed on the old tubercle. The secondary spines 

 do not lose their investment of skin, but they have no ligamentous attachment, 

 and their nerve-ring is not differentiated. The skin forming the basal swellings 

 of these spines is thicker than elsewhere, and the narrow basal ends of the ciliated 

 epithelial cells are separated by intercellular spaces. These are filled with the 

 refractile wander-cells, which readily escape when exposed to slight pressure; and 

 Hamannf 1 ) is mistaken in supposing that special gland-cells are present. The 

 pedicellariae of D. are of 2 types, 1. tridactyle and 2. analogous to the gem- 

 miform pedicellariae of other Urchins. The valves of the latter contain large 

 mucous glands, the opening of which at the apex of each valve is strongly ciliated. 

 The pedicellariae serve neither for locomotion, nor for holding on to fragments 

 of weed as suggested by Hamanni 1 ); for these functions are performed by the 

 tube feet. - The attachment of the peristomial membrane to the jaws and 

 pharynx is of such a nature as to cut off the interior of the test from all commu- 

 nication with the exterior round the teeth; while the jaw chamber is also shut off 

 from the body cavity by the membrane covering the lantern. Its 5 diverticula, 

 Stewart's organs, are not functional gills as supposed by Stewart. -- The first 

 curvature of the intestine passes into the second in Loven's radius III; and 

 the lacunae in the connective tissue layer disappear towards the rectum. The 

 ridges of the pharyngeal epithelium contain calyciforrn mucous cells of two kinds, 

 which are rare or absent in the furrows. Perrier's "intestinal siphon is absent in 

 D. ; but it is represented morphologically by a double series of folds along the 

 inner epithelium of the intestine, which mark the line of attachment of the internal 

 mesenteric fold and terminate at the commencement of the second curvature. 



The name Trichoelina paradoxa is given by BarrOJS to a parasite of Pterotrachea, 

 which maybe either an Echinoderm modified by parasitism or belong to a new group 

 altogether. There is a central disc and 3 short arms with an opening at the end 

 of each, leading into a separate digestive sac, and a water- vascular apparatus 

 consisting of a central chambered organ and a canal proceeding to each arm ; while 

 a sensory plate on each arm has a large nerve trunk connected with it. Ludwig 

 points out that T. is nothing but the detached head from the gemmiform pedicel- 

 laria of an Urchin. 



Duncan & Sladen adept the views of Carpenter respecting the homology of the 

 sur-anal plate of Salenia with the dorsocentral plate in the larvae of other Echi- 

 nozoa and with the radical plate of the Crinoid larva [see Bericht for 1885 I p 187]. 

 It is a primary embryonic plate, which grows as the test grows and consequently 

 remains outside the periproctal ring, not becoming separated from the basals by 

 the development of supplementary or periproctal plates. Its variations are described 

 in the species of Acrosalenia, the ambulacra of which contain compound plates, 

 each consisting of two 'primaries. Although the oldest genus of Saleniidae, it is 

 more distinct from the Cidaridae than either Peltastes or Salenia, neither of which 

 genera can have had an Acrosalenian ancestry. 



Gauthier describes and figures the variations in the arrangement of the genital 

 plates in Hemiaster, and concludes that it is impossible to base any generic cha- 

 racters upon this arrangement, as suggested by Poinel. 



According to Groom the am b ulacral plates of Pelanechinus are a modification 

 of the oligoporous Echinoid type, in which the two aboral primaries are represented 



