72 



PRINCIPLES OF ANIMAL BIOLOGY 



Fig. 59. 

 gramniatic 



-Hydra, dia- 

 representa- 



several kinds of cells. The bulk of that layer is made up of nearly cubical 



cells called the epithelial cells. Some of these epithelial cells, at the side 



toward the endoderm, are drawn out into long slender processes which 



serve both to contract, like muscles, and to convey impulses, like nerves. 



They are accordingly called neuromuscular cells 

 (Fig. 60). Between the bases of the epithelial 

 cells are numerous smaller rounded cells which, 

 from their location, are named subepithelial cells. 

 These give rise, at intervals, to very specialized 

 cells, the cnidohlasts, which travel toward the 

 surface of the ectoderm and produce within them- 

 selves a threadlike stinging apparatus called a 

 nematocyst. As the nematocysts are consumed 

 in attacking other animals or in defending the 

 Hydra, other cnidoblasts migrate to the surface 

 and produce new stinging threads. At the foot 

 of the animal the epithelial cells have the ability 

 to produce a sticky substance by which the body 

 is made fast to other objects and may therefore 

 be called gland cells. 



Hydra has also reproductive cells, which are 

 included in the ectoderm layer and which at inter- 

 vals develop into the mature cells, eggs and 



spermatozoa. The former, which are the female cells, raise the ectoderm 



into a rounded lump called the ovary (Fig. 59ov) ; the latter, the male 



cells, elevate the ectoderm into a conical mound called the testis (ts). 



Hydra also reproduces by buds (Fig. 596 1, 62), into which all the various 



body cells in the region of the bud enter. 

 It is thus apparent that Hydra, like 



Pleodorina and Volvox of the preceding 



section, possesses germ (reproductive) and 



somatic (sterile) cells. The existence of a 



budding process in Hydra, by virtue of 



which the somatic cells may share in the 



production of new individuals, does not 



alter the fundamental contrast between one 



class of cells which retain the typical mode 



of reproduction and another class of cells 



which have lost that power. Unliko> Pleodorina and A'olvox, however, 



Hydra has not stoi)ped with this one differentiation. It has gone farther 



and differentiated its somatic cells into five or six different kinds. 



Parallel between Foregoing Series and Individual Development. — 



Some biologists have favored tiie foregoing series of colonial i)i'otozoa 



tion of a lengthwise sec- 

 tion, bi, b2, buds in dif- 

 ferent stages of growth; 

 ec, ectoderm; en, endo- 

 derm; /, foot; gvc, gas- 

 trovascular or digestive 

 cavity or coelenteron; 

 TO, mouth; ov, ovary; t, 

 tentacle; ts, testis. 



Fig. go. — Neuromuscular cell 

 II.Nilra. (From Schneider.) 



