224 THE HOLOTHURIOIDEA 



the "biscuit" spicules are disposed in a thick inner layer 

 (Fig. II. 2 and 3, ta, b). The spicules are deposited by cells in 

 the cutis, and develop first as small rods which become branched 

 at each end ; by the increase of the number of branches and their 

 repeated fusion arise the innumerable varieties of spicules found in 

 different species of Holothurians. These calcareous bodies are of 

 great value to the systematist in classifying the smaller groups, 

 such as genera and species. 1 Although their general characteristics 

 are fairly similar within the several families, the different shapes 

 of spicules are not sufficiently constant to be used as diagnostic 

 characters of such large divisions. 



Very characteristic of the Holothurians is the calcareous ring 

 formed of radial and interradial pieces surrounding the oesophagus. 

 Occupying the same position as " Aristotle's lantern " in the Echi- 

 noids, it may possibly be homologous with that organ. 



A well-developed muscular system is present in the body-wall, 

 whereby the animal can move and alter its shape. The longitudinal 

 muscles, generally paired radial bands, often form special retractors 

 for the withdrawal of the anterior region of the body. In the 

 Synaptidae alone the circular muscles are not interrupted at the radii. 



Although the nervous and water-vascular systems are distinctly 

 built according to the pentagonal radiate Echinoderm type, yet the 

 latter system is generally modified in relation to the very frequent 

 differentiation of a dorsal surface occupying the three upper interradii, 

 and of a flattened ventral surface or creeping sole occupying the 

 two lower interradii. The ambulacral appendages, the podia, may 

 become modified from typical sucking-feet into pointed papillae. 

 Either the tube-feet or the papillae may extend over the entire inter- 

 radial space. On the other hand, both tube-feet and papillae may be 

 absent (Molpadiidae, Pelagothuriidae, and Synaptidae), and even the 

 radial canals may disappear in the adult (Synaptidae). The oral 

 tentacles, so characteristic of the Holothurians, are no doubt modified 

 ambulacral appendages homologous with the podia. They vary 

 greatly in number and shape, and are of great taxonomic value, 

 being almost invariably more or less peltate in the Holothuriidae and 

 Elpidiidae, arborescent in the Cucumariidae, and digitate or pennate 

 in the Molpadiidae and Synaptidae. 



The Polian vesicle is usually single, and situated in the left 

 ventral interradius ; but there are sometimes more than one. The 

 stone-canal, generally single, lies in the median dorsal mesentery. 

 Although in some of the Holothuriidae, Elpidiidae, and Pelago- 

 thuriidae, and in the larvae of other forms, the stone -canal retains 

 its opening to the exterior in the median dorsal line, in most Holo- 

 thurians this condition is modified, in that the primitive madreporite 



1 The structure of the spicules is liable to alter during the lifetime of the 

 individual (Herouard, Mitsukuri, Ostergren). 



