400 GENERAL ZOOLOGY 



pharynx, which usually can be thrust from the mouth as a 

 movable muscular proboscis (Fig. 228). 



SUBCLASS 1. ACOELA. A mouth, with or without a probos- 

 cis, is present, but the remainder of the alimentary canal is 

 absent. Eyes are usually lacking. There is a statocyst in 

 the dorsal surface over the cephalic ganglia. 



Example: Childia spinosa, marine form, 1.4 mm. in length. 



SUBCLASS 2. COELATA. An intestine is present. 



Order 1. Rhabdocoelida. The intestine is an unbranched tube, 

 in some cases, slightly lobulated. Found in marine and fresh 

 water and on land (Fig. 228, A and B). Some reproduce 

 asexually by terminal budding (Fig. 228, B). 



Examples: Dalyellia, a fresh-water American form; Micro- 

 stomum, both fresh- and salt-water genus. 



Order 2. Tricladida. Commonly known as planarians and 

 characterized chiefly by an intestine with three main branches, 

 one extending forward and two backward from the pharynx. 

 Each main trunk gives off diverticula with separate branches. 

 A pair of eyes are usually present (p. 192) in the head end. 

 On either side in front of the eyes is a sensitive lobe or auricle. 

 Example: Euplanaria maculata, a fresh- water form (Fig. 228, 



C). 



Order 3. Polycladida. The intestine has many branches 

 ramifying to all parts of the body. Eyes, otocysts, tentacles, 

 and tactile organs are well developed (Fig. 228D). 



Example: Planocera inquilina, found as a commensal form 

 in the branchial chamber of a large marine snail, Busycon. 



CLASS II. TREMATODES. Flukes. Exclusively parasitic. 

 Cilia are absent, or present only in larval stages. Usually the 

 mouth is subterminal and the intestine is bifurcated. Hooks 

 and suckers are found in ectoparasites for attachment to the host, 

 but suckers only in endoparasites. The name of the class, 

 Trematodes, refers to the presence of suckers (trema, hole). 

 Eyespots occur in ectoparasites, but only in the larva of 

 endoparasites. 



Order 1. Monogenea. For the most part are ectoparasites on 

 aquatic animals, but some change to endoparasitism. There is 

 but one host. The organs for attachment, at the ends of the 

 body, are well developed. Some have small pores. 



