35O LOUIS MAX HICKERNELL. 



OBSERVATIONS, i. On Entire Animals. 



A. Anatomy of P. roseola. Janson ('93) has carefully con- 

 sidered the anatomy of the Philodinidae and I find few inac- 

 curacies in his statements as applied to P. roseola. It will be 

 necessary, however, to briefly describe the organ systems and 

 their relationships in order to arrive at a thorough understanding 

 of the changes in form and rearrangement of parts incident to the 

 desiccation process. 



The body of P. roseola, at first sight, suggests that of a seg- 

 mented worm. Its segmentation (which affects only the integu- 

 ment and accessory structures) is, however, not so regular but 

 that the body is easily divided into head, trunk and foot. Its 

 outer integument is formed of a thin, more or less transparent 

 cuticle beneath which is a granular plasma layer, the hypodermis. 

 In the hypodermis are deposited pigment granules. These are 

 absent in the very young animal and occur in varying number in 

 the adult. The cuticle is soft and flexible, easily lending itself 

 to the telescoping and folding so characteristic of this Philodina. 



The hypodermis is a syncytium which contains nuclei distri- 

 buted evenly through it. There are, in the mid-body region, 

 longitudinal folds of the skin which are always present. Tem- 

 porary cross foldings occur as a result of the vigorous movements 

 of the animal. Externally the skin lacks spines or protuberances 

 of any kind. 



The head, whose most prominent structures are the trochal 

 discs, extends as far back as the forward margin of the mastax. 

 When the trochal discs are folded in, a blunt, ciliated projection 

 is seen at the anterior end. This is the proboscis and it is used 

 for tactile purposes as well as to aid in the "measuring-worm" 

 type of locomotion so characteristic of the Bdelloidese. The 

 rotating organs consist of a double row of cilia, broken in the case 

 of the anterior row, in the ventral part of the head. The more 

 posterior ciliary wreath is made up of shorter elements and is 

 directly continuous with the cilia which line the pharangeal 

 cavity. The hypodermis in the head region at the base of the 

 trochal discs is thickened, probably to furnish additional support 

 for the rotating apparatus, and in some specimens the nuclei 

 of this layer can be seen near the bases of the lines of cilia. 



