CQELENTERATA 



53 



In addition to this differentiation of the 

 cells into layers, there is usually also a 

 differentiation into somatic or body-cells 

 and germ- or reproductive-cells ; the latter 

 are of two kinds, the male cells or sperma- 

 tozoa (Fig. 34), and the female cells or ova 

 (Fig. 35); so that while reproduction occurs 

 in some cases by division and by budding, it 

 generally takes place by means of fertilized 

 ova, a process which does not occur typically 

 in the Protozoa, but is suggested in such 

 types as Volvox, where the macrospores and 

 microspores may be compared to ova and 

 spermatozoa respectively. 



There are a few simply organized animals 

 whose structure is such that their position 

 in the system of classification is very doubt- 

 ful. An intermediate subkingdom, the 

 Mesozoa, is sometimes made for them, but 

 they are of little importance for the ele- 

 mentary student (Fig. 36). 



We shall divide the Metazoa into six 

 types, though these are far from being of 

 equal importance, and several of them are 

 necessarily divided into subtypes. 



Fig. 36. Rhopahna giar- 

 dii, J , one of tlieMesozoa ; 

 parasitic in the body-cavity 

 of Ophiuroidea; greatly 

 enlarged. ( From Parker 

 and Haswell's Text-book.) 



TYPE II. CCELENTERATA 



The Ccelenterata (Gr. kgIXos, hollow, and evrepov, intestine) 

 constitute the simplest type of the Metazoa. In their least 

 modified condition, the body is saclike, with an opening at 

 one end. The wall of the sac consists of ectoderm on the 

 outside, entoderm on the inside, and between these a base- 

 ment membrane or a mesenchyme, — a true mesoderm does 

 not occur in this group, and thus a body-cavity or ccelom is 

 absent. The body contains a single cavity, bounded by the 

 entoderm, and this is generally known as the ccelenteron ; it 

 functions as a digestive tract. Some of the cells of the body 

 are modified into muscle-cells, others into nerve-cells, and others 



