148 TEXTBOOK OF ZOOLOGY 



are seasonal reproductive organs. Both ovaries (female gonads) and 

 testes (male gonads) may be formed on a single individual, but they 

 are usually seen on separate individuals. If these projections are 

 conical and located nearer the tentacles, they are testes or sper- 

 maries; if they are more nearly knoblike and are located nearer 

 the base, they are ovaries. This animal possesses radial symmetry, 

 but it is arranged with an axis of polarity from basal disc to hypo- 

 stome, which is essentially equivalent to what is called the ventro- 

 dorsal axis of more advanced forms. All of the metazoans have a 

 primary axis. Sedentary and sessile animals very commonly have 

 radial symmetry, while the motile or free-living organisms tend 

 toward bilateral symmetry. 



Internal Anatomy 



Another feature of the organization of this animal is the diplo- 

 blastic structure which consists of two layers of cells or the germ 

 layers surrounding an internal space, the gastrov oscular cavity or 

 enteron. These are studied on stained sections. The outer one is 

 the ectoderm, which is thinner and is composed of four types of cells. 

 The most numerous ones are typically cuboidal in shape and serve 

 both as contractile units and as the general external surface of the 

 body; they are appropriately called epitheliomuscular cells. Each 

 of these cells consists of a polyhedral outer or epithelial portion and a 

 basal portion which is drawn into one or two long, slender, fibrils 

 which extend in a direction parallel to the length of the animal. 

 These cells contract to shorten the length of the animal. Interspersed 

 occasionally among these cells are the larger cnidohlasts in which 

 develop the neniato cysts, stinging cells or nettle cells. These are dis- 

 tributed over all the body except the basal disc, but they are much 

 more numerous near the distal part of the column and on the ten- 

 tacles. The nematocysts are usually contained in little raised 

 tubercles in the ectoderm. Each tubercle contains a large barbed one 

 and several of a smaller variety. Four different kinds have been de- 

 scribed. Since the large barbed type is the most conspicuous, it will 

 be described here. In the cnidoblast the nematocyst appears as a 

 sac of fluid within which is inverted a barbed stalk with a coiled 

 thread attached. Projecting out of the superficial surface of the 

 cnidoblast is a triggerlike process called the cnidocil, which when 

 chemically stimulated causes the cnidoblast to discharge the nemato- 



