ORGAN- SYSTEMS COMPARED 



The coelom in all but the lowest Coelomocoela has by its large 

 development led to a very marked separation of the body-wall and 

 the gut-wall, and a consequent independent development of elaborate 

 SYSTEMS of MUSCULATURE in each of these superimposed regions. 

 In the Enterocoela there is no separation of body-wall-musculature 

 and gut-wall-musculature (nor in Platyhelmia, Nemertina, and 

 Nematoidea among Coelomocoela). 



The SENSE-ORGANS of the Enteroco3la attain in some cases a 

 high degree of complexity (optic and auditory structures), but 

 the nerve tissue remains even in the highest to a large extent 

 diffuse, and in the form of a widely scattered network, though 

 ring-like concentration corresponding to the form of the body is to 

 some extent found. In the Coelomocoela, even among the lowest, 

 a concentration of the nerve ganglion cells to form the CENTRES of 

 a NERVOUS SYSTEM is observed. Various steps in this concentra- 

 tion in the form of longitudinal cords may be observed in lower 

 and higher Ccelomocoela, tending to extreme concentration of the 

 nerve-ganglion-cells, and the protection and special nourishment 

 of the brain and nerve cord so produced. 



BRANCHIAL RESPIRATORY ORGANS are frequently developed as 

 feather-like outgrowths or other modifications of the surface in 

 Coelomocoela. The blood-vessels are distributed in these branchiae 

 and there receive oxygen, and liberate carbonic acid. In the 

 Enterocoela, the absence of a vascular system is accompanied by 

 the absence of special branchial organs. 



In GENERAL FORM and SYMMETRY, as well as in the manifesta- 

 tion of merogenesis, or repetition of parts, the Enterocoela and 

 Coelomocoela differ greatly. In both a primary bilateral symmetry 

 can be (with a few exceptions among the Enterocoela) detected. 

 But in the Enterocoela this is masked by a dominating tendency 

 to radial symmetry. Such masking of the more primitive bilateral 

 symmetry is rare in Coelomocoela, where, however, it is exhibited 

 by most of the Echinoderma. 



MEROGENESIS. The Enterocoela frequently give rise to lateral 

 buds, and so to arborescent growths, consisting of many individuals. 

 The Ccelomocoela more rarely produce lateral buds (Polyzoa, 

 Tunicata). The Coelomocoela often give rise to chains of complete 

 or incomplete individuals by growth, along the oro-anal axis, and 

 partial or complete division at right angles to that axis (meta- 

 meric segmentation). An apparently similar process is seen in the 

 segmentation and division of the Scyphistoma polyp at right angles 

 to the oro-aboral axis. 



The exact historic relationship of metameric segmentation and 

 repetition of parts in the Coelomocoela to a previously complete 

 production and separation of metameric " buds " or new individuals, 

 requires special consideration in each group of animals in which 



