176 INVERTEBRATA CHAP. 



that the slime glands, which open on the oral papillae and secrete 

 the silk with which Peripatus spins its web, are of ectodermal origin, 

 as are also the crural glands which open on the inner side of the 

 legs external to the openings of the excretory tubes. The tracheae 

 develop from simple ectodermal ingrowths which arise very late in 

 development. The eyes are simple and formed like the eyes of Mollusca, 

 'as vesicles which become closed off from the exterior. A cuticular lens 

 is secreted by the cells of the anterior wall, and the cells forming the 

 posterior wall become the visual cells. The whole organ resembles the 

 "ocellus "of an Insect larva and the cuticular lens inny.be compared 

 to the "glass body" of the latter (see Fig. 219). 



From this necessarily brief and condensed sketch of Sedgwick's 

 results, it will be seen that the change from the annelid to the 

 arthropod type of structure must have been accompanied by a 

 suppression of the coelom and an enlargement of the blood-spaces, 

 the latter forming the functional perivisceral cavity of the adult, 

 while remnants of the former persist in the end-sacs of the excretory 

 organs and the cavities of the generative organs. 



This change was also accompanied by an intensification of the 

 secretion of cuticle, and it is just conceivable that this intensifica- 

 tion of the secretory powers of the ectoderm entailed the other 

 changes which supervened. If chitin be allied to uric acid, as has 

 been asserted, and if the production and casting off of chitin can be 

 likened to nitrogenous excretion, then we may understand how the 

 coelomic wall, which had previously undertaken a considerable portion 

 of this function, might become relatively unimportant and might 

 tend to dwindle and disappear. 



In Peripatus the chitinous cuticle is thinner and more flexible 

 than in any other known Arthropod, and in no other Arthropod is 

 a continuous series of " uephridia " retained. In all others the cuticle 

 is thicker and the " nephridia " are reduced to one or a very few pairs ; 

 in some cases they seem to be absent altogether. Cuticle and 

 " nephridia " seem therefore to vary in development inversely to one 

 another, and since increase in cuticle seems to entail decrease in 

 " nephridia," it may well be that the same factor has led to the 

 decrease and disappearance of the coelom. 



