INTEGUMENT OF CCELENTEEATA. 103 



the oral surface of the body, or are scattered over it ; tliey are 

 sometimes produced on to lobate processes also. 



lu the Ctenophora, processes of no great size are also occasionally 

 present on the edge of the mouth in some families (Calymnidas, 

 Callianirida?) ; and there are large elevated lobate extensions of the 

 body, which we may regard as tentacular organs, although morpho- 

 logically they are different structures. Besides these, some genera 

 (Oydippidss) have " grappling-lines," which resemble the marginal 

 filaments in the Medusas, and correspond in position with the poles 

 of an interradial transverse axis : sometimes they are provided with 

 secondary appendages. 



Integument. 



§80. 



The most primitive characters of the integument of the Ccelen- 

 terata are seen in the Spongia3, where it is conrposed of the ecto- 

 derm, which is but slightly differentiated, and follows the various 

 changes of form in the endoderm, which limits the nutritive cavity. 

 The special characters consequent on this relation are referred to 

 below (§ 87). In the Physemaria) the cells of the ectoderm form a 

 syncytium. In the Porifera they may be sometimes seen to form a 

 thin layer (Halisarcina, Sycon). 



Among the Acalephas, the ectoderm undergoes differentiation 

 very early, so that the most external layer of cells, or epidermis, 

 which is distributed over the whole body, represents in most cases a 

 portion only of the primitive ectodermal layer. The investment 

 of cilia, which in the Spongiaa is limited to the earlier stages of 

 development, not only persists in the Acalephas during the so- 

 called larval stages, when it has a locomotor function, but is 

 frequently continued on into the later stages, when it is generally 

 limited to separate parts, e.g. the tentacles. 



As the body increases in size, the importance of the cilia, as 

 locomotor organs, disappears. In one class only — the Ctenophora — 

 do they retain this function, and they are then increased in size. In 

 the place of the general investment, as seen in the larva, structures 

 resembling cilia are disposed in longitudinal rows, and by increasing 

 in length and breadth become converted into movable swimming 

 or rowing plates. The plates are attached to the body by their 

 broader base ; it is at this point only that contractility, dependent 

 on the voluntary influence of the animal, is manifested ; the rest and 

 larger portion of the plate seems to be rigid. There are generally 

 eight rows of these plates, which act as steering organs ; but in many 

 Ctenophora there are only four rows (Cestum). The urticating 

 capsules (nemocysts) are special differentiations of the epithelial 



