ORGAN -SYSTEAfS COMPARED 



6. Chief Organs and Organ-systems of Enteroc(ela and 

 CcELOMOCiELfi. — Leaving out of consideration special locomotive 

 and prehensile mechanisms, and confining our attention to 

 differentiations of structure corresponding to important physio- 

 logical processes in the animal economy, we note in comparing 

 Enterocoela and Ccelomocoela that it is by no means merely in 

 the possession of the ccelom that the latter grade rises above the 

 former. In all but the simplest Ccelomoccela (the Platyhelmia 

 and some few minute forms) we find a blood-vascular system, 

 consisting of main arterial and veinous trunks connected by rami- 

 fying capillaries, present. In rare instances only are the fine 

 capillaries absent, and their place taken by larger trunks. The 

 essential element of this system is a modification of a primary 

 tissue similar to the embryonic connective tissue of Vertebrata. 

 Its distinctive character is that the constituent cells form elongated 

 fibre-like groups, branching and constituting a reticulum, whilst 

 at the same time the cell -substance, instead of giving rise 

 to fibrillar skeletal material, becomes liquefied axially. Thus 

 tubes consisting of rows of elongated nucleated cells are formed 

 containing a highly organised liquid, which is often coloured red 

 with hnemoglobin, and contains the nuclei of disintegrated cells, 

 which were the sources of the hannoglobinous fluid, as in 

 ChcVtopoda and some Mollusca (Planorbis) and some Arthropoda. 

 On the other hand, the fiuid may be colourless, whilst in it fiout 

 hajmoglobinous corpuscles, as in Vertebrata, some Mollusca (Solen 

 legumen. Area), and some Echinoderma, or the fluid may not only 

 itself be colourless but contain only colourless floating corpuscles 

 (most Molluscs, Arthropods, and Echinoderms). 



Eenal excretory organs specially developed in the form of 

 sacs (renal sacs) and tubes (nephridia) are found in the Coelomo- 

 coila, whilst in the Enterocala, although some cells or even cell 

 groups appear to have a renal excretory function — that is to say, 

 to be concerned in the elimination of nitrogenous waste — there are 

 no definitely constituted renal organs. 



The regions and glandular appendages of the alimentary 

 TRACT are, except in the Platyhelmia, very differently developed 

 in the Enterocoela and Ccelomocoela. A stomodanim (y-o/xa, the 

 mouth, and oSaior, adj. form of oSos, a road) results from a tube- 

 like in-pushing of ectoderm in the first formation of the mouth in 

 higher Enteroca»la. In the Ca4omocu'la we not only get a 

 stomod;vuni, but an ectodermal proctod;\ium (r/awK-rds- and u^alov) 

 is similarly formed in connection with the anus, which is rarely 

 absent in that grade, and never present in the lower. 



Paired digestive glands of various kinds, having the form of 

 saccular outgrowths of the gut, are present in most Ca'lomoca?la, 

 and never found in Enteroctvla. 



