DEYELOPMEN . 213 



The tissues (which are composed of cells) are generally arranged in 

 two or three layers, of which the external layer is known as ectoderm 

 and forms the outer skin, while the internal layer, the endoderm, 

 lines the gastric cavity. 



Between the two there is developed a delicate homogeneous sup- 

 porting membrane or a stronger layer of connective tissue, in which 

 the skeletal elements are developed. This intermediate layer is 

 known as the mesoderm. The skeletal formations present great 

 variations in structure and arrangement. 



Muscles are formed in the deeper part of the ectoderm as cell- 

 processes (the so-called neuromuscular fibres), but often penetrate 

 within the mesoblast as independent cell structures. Sense epi- 

 thelium, nerve fibrilloe, and ganglion cells also appear as differentia- 

 tions of the ectoderm. The endoderm cells, on the other hand, often 

 bear cilia, and are principally concerned in the processes of digestion 

 and secretion. 



A sexual reproduction by fission and gemmation is prevalent in 

 these animals, constituted, as they are, on the whole of homogeneous 

 tissues. If the individual forms so produced remain united, they give 

 rise to the colonies which are so widely distributed amongst the 

 Polyps and Sponges, and which, by the continual multiplication of 

 their individuals, may in course of time attain a very considerable 

 size. But we also meet everywhere with the sexual reproduction, in- 

 that ova or spermatozoa are produced in the tissues, usually in the 

 region of the gastrovascular cavity, in a definite portion of the body. 

 As a rule, the ova come in contact with the spermatozoa away from 

 the place where they are produced ; either within the body cavity 

 or outside the parent body, in the sea-water. In a few cases only do 

 both the sexual elements originate in the body of the same indivi- 

 dual, as, for example, in many of the Spongiaria, some Anthozoa, 

 and in the hermaphrodite Ctenophora. As a rule, in the colonies of 

 Anthozoa the monoacious arrangement of sexes obtains, the indivi- 

 duals of the same stock being partly male, partly female. Some are 

 dioecious, e.g. Veretillum, Diphyes, Apolemia,. 



The development of the Ccdenterata for the most part consists of 

 a metamorphosis. The just hatched young differ from the sexual 

 animal in the form and structure of the body, and pass through 

 larval stages. The greater number of them leave the egg as 

 ciliated larvae, which resemble somewhat an Infusorian i-n external 

 appearance. They acquire a mouth, body cavity, and organs for 

 obtaining food, either during their existence as free larva?, or after 



