314 ESSENTIALS OF ZOOLOGY 



(free, oral, or anterior) end of the column is much stouter than 

 the proximal (attached, aboral, or posterior) end, but in H. viridis- 

 sima there is only a slight tapering toward the basal end. Attached 

 around the free end of the column is a circlet of from four to seven 

 fingerlike tentacles, which extend free in the water. Tentacles may- 

 stretch out to be slender threads five to seven centimeters in length. 

 They are very useful either singl}^ or as a group in capturing and 

 delivering food to the mouth. The mouth is located in the center 

 of the distal end of the column and is surrounded by the tentacles. 

 This conical elevation between the bases of the tentacles in which 

 the mouth is located is called the hypostome. The mouth when 

 closed and viewed from the top looks something like an asterisk. 

 From the side it appears simply as an indentation or notch in the 

 conical end of the hypostome. The proximal or attached end termi- 

 nates in a basal disc or foot, w^hich secretes an adhesive substance 

 which helps the animal in attaching to objects. From one to several 

 huds are often found on the sides of the trunk, and these occasion- 

 ally bear buds before the first is separated from the original parent. 

 Buds are lateral outgrowths of the column and are found when the 

 animal has favorable living conditions. Budding usually occurs at 

 about the middle of the body in //. viridissima. Occasionally there 

 may be observed rounded projections on the side of the column which 

 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. 

 Sedentary and sessile animals very commonly have radial symmetry, 

 while the motile or free-living organisms tend toward bilateral sym- 

 metry. 



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 gastrovascidar 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 



