TENTACULATA 



151 



It will be seen that a longitudinal plane passed through the body 

 which includes the mouth and stomach divides the body into two sym- 

 metrical halves; a transverse plane, on the other hand, reveals a radial 

 type of structure. 



The gastrovascular space consists of a complex system of narrow 

 tubes (Fig. 248) lined with entoderm which join the inner end of the 

 stomach, and communicate with the outside also 

 by means of either one or two pores at the aboral 

 end of the body. Eight of these tubes which lie 

 immediately beneath the eight longitudinal bands 

 of cilia form the most important part of the sys- 

 tem. The space between these gastrovascular tubes 

 and the outer ectoderm is filled with the soft jelly- 

 like mesenchyne which differs from the mesoglea 

 of the other coelenterates in that it arises as the 

 result of the proliferation of definite cells during 

 the early development of the animal; in it are 

 nuclei and muscle fibers. 



All ctenophores are hermaphroditic, the gonads 

 consisting of a pair of bands, one male and the 

 other female, which lie side by side against the 

 outer wall of the main longitudinal canals of the 

 gastrovascular space, the genital products reaching 



the outer sea water through the mouth. The young animal passes 

 through a complex metamorphosis before reaching the adult condition ; 

 but there is no alternation of generations. Certain genera may exhibit 

 paedogenesis, reproducing in the larval stage, and again as adults. 



Ctenophores are common marine animals, often occuning in enor- 

 mous schools. They are noted for their delicacy and beauty, the rapidly 

 vibrating combs refracting the light and showing a rapid play of 

 changing colors. They are also often highly phosphorescent at night. 

 Their food consists of crustaceans, fishes, and other small animals, often 

 including their own kind. The subphylum contains two classes and less 

 than 100 species, 21 of which occur off the Atlantic coast. 



Key to the classes of Ctenophora: 



Oi Either tentacles or oral lobes present 1. Tentaculata 



a^ Tentacles and oral lobes absent 2. Nuda 



Fig. 248 — Diagram 

 of a ctenophore, show- 

 ing the canal system 

 (Mayer). 1, aboral end 

 with statocyst ; 2, lon- 

 gitudinal canals ; 3, 

 oral end of body ; 4, 

 mouth; 5, stomach. 



Class 1. TENTACULATA. 



A pair of long tentacles present, in certain cases in the larval stage 

 only, oral lobes being then present in the adult: 3 orders. 



