Ill PLATODES—BOBY FORM 137 



solid tentacle with short lateral branches. In the neighbourhood of the tentacle on 

 each side is found an aperture leading into a system of canals which branch in the 

 body parenchyma, and which the discoverer of Ctenoplana compares with the water 

 vascular system of the Platodes. Under the body epithelium lies a basal or skeletal 

 membrane, under this a layer of longitudinal muscles, and under this again a layer 

 of ti'ansverse muscles. Besides these there are dorso-ventral muscular fibres branched 

 at both ends, which adhere on one side to the dorsal, and on the other to the ventral 

 basal membrane. There are special bundles of muscidar fibres for the protrusion and 

 withdrawal of the cDiated plates. 



These two forms agree with the Ctenophora chiefly : 



1. In the possession of an aboral sensory body. 



2. In the possession of 8 rows of ciliated plates [Ctenoplana). 



3. In the possession of feathered tentacles. 



4. In the general structure of the body. 



Ccsloplana and Ctenoplana are not yet bilaterally symmetrical. The chief axis runs, 

 as in the Ctenophora, from mouth to sensory body. It is very much shortened. The 

 lateral plane runs through both tentacles ; the median or sagittal plane stands at 

 right angles to it. Each of these planes divides the body into 2 similar halves. 

 If such forms were always to move forwards in the direction say of its median plane, 

 and if at this end special sensory organs were to develop, or the sensory body come 

 more forward, they would become bilaterally symmetrical. Only one plane, viz. 

 the median plane, would divide the body into 2 exactly similar halves. We could 

 then not only distinguish upper and lower, but anterior and posterior regions of the 

 body. 



Ccdoplaim, and Ctenoplana agree with the Polyclada : — 



1. In the flatly compressed form of body, and in the capacity of moving forward 

 by creeping. 



2. In the general cUiation of the body. 



3. In the possession of a skeletal membrane {Ctenoplana). 



4. In the possession of a dermal musculature, consisting of a. longitudinal and a 

 circular layer. 



5. 'In the possession of dorso-ventral muscle fibres branched at both ends. 



6. In the general arrangement of the gastro-canals. 



7. In the possession of 2 (in Polyclada, however, unfeathered) dorsal tentacles 

 and of a dorsal nerve centre (?). 



8. In the possession of a water- vascular system [Ctenoplana?). 



The peculiarities mentioned under 1-5 may be considered as due to the creeping 

 mode of life. 



Cceloplana and Ctenoplana are distinguished both from the Ctenophora and the 

 Polyclada by the want of an ectodermal oesophagus, — at least no such cesophagus 

 has been described. 



II. The Body Form. 



Most of the Platodes are, as their name indicates, more or less 

 flat. The Polyclada are leaf-shaped. In these all intermediate forms 

 between the broad oval and the long ribbon occur. The Trklada are 

 mostly long, lancet-shaped, with dorsal surfaces slightly arched. Among 

 the land Triclada forms occur of great length. In the Bhabdoccda 

 great variety prevails ; there are flat, disc-shaped, plano-convex, and 

 often spindle-shaped animals. Among the Trematoda the ectoparasites 

 and a few small endoparasites (e.g. Distoma hepaticum, D. lanceolatum) 



