ALIMENTARY CANAL OF CCELENTEEATA. 113 



by dermal pores only. Tlie primitive enteric cavity in them, as in 

 the Leucones, where it loses its layer o£ flagellate cells (endoderm), 

 also loses its nutritive functions, which are confined to the radial 

 tubes. These latter seldom remain free, but generally unite either, 

 in part or completely, by their walls with an important layer which 

 surrounds the primary enteric cavity. A system of canals, which 

 are only invested by ectoderm, is formed out of the space between 

 the radial tubes when these only fuse in part. This form may be 

 seen in the Sycones, among the Calcareous Sponges. 



Innumerable modifications, including individual varieties, are 

 present within the range of one type-form. The primary enteric 

 cavity is altered in character by the formation of diverticula, as well 

 as of septa, or trabeculte, and may even be completely atrophied, 

 while the canal system arising from it is developed ; this phgenomenou 

 (lipogastria) is not uncommon in the Fibrous and Siliceous Sponges. 

 A similar atrophy may even affect the mouth (lipostomia) without 

 affecting the stomach; in such a case the dermal pores take on 

 the function of ingestive canals; or numerous small spaces, as in 

 Euplectella^ arise in the place of the mouth. 



§ 88. 



The form of the gastric system is greatly afFected by the forma- 

 tion of stocks, a process due partly to the concrescence of free persons, 

 and partly to budding. The union, according to the degree of its 

 development, may then simply cause communication between the 

 stomachal cavities, which persist for each person (Fig. 41), or lead to 

 a complete union of the cavities; in which case the mouths too 

 may undergo reduction, or become reduced to one, which likewise 

 may disappear. 



A special system of cavities (inter-canal system) also ai'ises from 

 the formation of stocks. This system is formed from the spaces 

 which persist between the unconnected parts of the persons, or the 

 anastomosing branches, of the body ; this, like the system mentioned 

 above in the Sycones, is bounded by the ectoderm only, and is thus 

 essentially distinguished from the gastric system. It is remarkable 

 for considerable irregularities in its arrangement, and forms also 

 wider spaces, which deceptively resemble a stomachal cavity in that 

 they possess a mouth. 



From all these arrangements a significant change of function 

 in different parts is seen to accompany the change of form in 

 the Spongiee. The physiological activity of the digestive cavity 

 is not only shared by the secondary canals which arise from it, 

 but even passes away from it altogether, or is limited to separate 

 portions of it, when consequently the stomach sinks, physiologically 

 speaking, to a lower grade. On the other hand, an important func- 

 tion becomes localised by this change of the primitively subordinate 

 portions of the canal system, and even the original surface of the 

 body of the sponges gets a higher significance, in virtue of its 

 serving as the fining of the inter-canal system. Everything distinctly 



