THE APODOUS HOLOTHURTANS 61 



formis the second quintet of tentacles appear all at the same time. Probably 

 the 10-tentacled larva (Plate VI, fig. 4) is characteristic of all Synaptids. In 

 the viviparous species the auricularia seems to be wanting and the pupa stage 

 is passed through quickly, nor are there any sharp lines between the different 

 steps in the process of development; the pentactula and 10-tentacled larvae 

 are, however, plainly shown. In other respects there is little essential differ- 

 ence between the development of the oviparous and viviparous species. In the 

 larvfc of Synaptula hydriformis, however, there are peculiar glandular organs 

 in the ectoderm, which may have some use in absorbing food material from 

 the body-cavity fluid of the parent. Perhaps these glandular organs are iden- 

 tical with the "contractile rosettes" of certain other Synaptids. (See Becher, 

 : 06. ) In species with twelve tentacles, the eleventh and twelfth arise, one in 

 each of the two lateral, dorsal interradii ; nothing is known of the development 

 of the additional tentacles in species with more than twelve. 



In conclusion, the organogeny of the Synaptids may be stated as follows: 

 Ectoderm: The epithelium of the entire body surface, including the sensory 

 epithelium of the tentacles, the entire nervous system, the sense-organs, the 

 anterior part of the oesophagus and possibly the posterior part of the rectum 

 arise from the larval ectoderm. Mesoderm: The calcareous concretions, the 

 calcareous ring, the pigment-cells and all cartilaginous and connective tissues 

 arise from the mesenchyme cells of the larva, which in turn arise from the 

 archenteron ; there is no satisfactory evidence that any muscles are formed 

 from these cells. Endoderm: The alimentary canal (except the ectodermal por- 

 tions already mentioned), the water- vascular system, the entire musculature, 

 the ciliated funnels, the blood-vessels and corpuscles, the reproductive organs, 

 and all internal epithelial tissues arise from the larval endoderm, either di- 

 rectly from the archenteron or from the hydroco3l and coelomic pouches to 

 which it gives rise. 



PHYSIOLOGY. 



Comparatively little work has yet been done in studying the functions of 

 various organs of Holothurians or indeed of any Echinoderms, and particu- 

 larly is this true of Synaptids. Most of our knowledge of the physiology of 

 these animals is the result of casual observation or chance discoveries made 

 while some problem of morphology or embryology was under investigation. 

 What little is known may be conveniently grouped under the following heads : 



MOTION. The movements of Synaptids are mainly concerned with the col- 

 lection of food and the avoidance of enemies, and are dependent upon the 

 fluids within the body-cavity and water-vascular system quite as much as upon 

 the muscles ; the calcareous particles in the body-wall also play an important 

 part in the locomotion of many species. In the search for food, the animal is 



