cxl Introduction. 



amongst the Crustacea; whilst the winter ova are gamogenetic. 

 The embryos are developed from the entire yolk, without the 

 formation of any primitive streak ; and they do not ordinarily go 

 through any metamorphosis. 



They may be either solitary or social. They are mostly inhabit- 

 ants of fresh water, but some are marine ; a few are found living 

 as parasites upon animal and vegetable organisms. They possess 

 great powers of recovery after desiccation. 



Class, Platyelminthes. 



Vermes, with more or less completely flat, leaf-shaped, or tongue- 

 shaped bodies, devoid both of external annulation and of internal 

 perivisceral cavity. They are ordinarily aproctous, and, though 

 possessed of complex reproductive organs, hermaphrodite. Meta- 

 genesis, with ' alternation of generations,^ is very common in this 

 class. They are divisible into three orders — the Turhellaria, the 

 Trematodes, and the Cestodes. Of these the two latter are para- 

 sitic in habit, as is indicated by their possession of organs of adhe- 

 sion in the shape of suckers or of hooks, or of both, and by their 

 non-possession of cilia in their adult parasitic life ; whilst the 

 Turhellaria have a ciliated integument, whence their name is taken, 

 and never possess either suckers or hooks. Even in the richly 

 ciliated Turhellaria^ the muscular layers of their body-walls are 

 their active locomotor organs. There are three of these muscular 

 layers in most Platyelminthes, the innermost, however, is said to 

 be wanting in certain Turbellarians ; like the outer layer it takes, 

 when present, a direction more or less completely at right angles to 

 the long axis of the body. Layers of granular glandular substance 

 are observable in the more deeply-lying of the cortical layers of the 

 integumentary system of all members of this class ; and crystals of 

 carbonate, and of phosphate of calcium are very commonly found in 

 the cortical, and more sparingly in the more centrally placed por- 

 tions of the body, especially of the Trematodes and Cestodes, and 

 occasionally also of Turhellaria. Chitinous hooks and spines are 

 very commonly present, but there is never any extensive induration 

 of the integument in this Class, either by chitinous or calcificatory 

 deposit. 



The Cestodes, which are always entoparasitic in their sexual 



