198 THE EVOLUTION OF THE METAZOA 



form of the activity of skin glands, the secretion of the mucus, 

 has later been developed into various other more specialized 

 secretions. As early as in solitary Anthozoa we can find on 

 one hand a more local (at the flattened sole of the foot) secre- 

 tion of a glutinous and therefore thicker slime, and on the 

 other hand (especially in Ceriantharia) a glutinous and tubular 

 secretion whose duty is to protect the animal body. The for- 

 mation of the periderm also belongs here, with the difference 

 that here slime has become solid matter. If we disregard the 

 formation of slime, which is connected with the feeding me- 

 chanism of the animal, we find in scypho- and hydro-polyps 

 the gluti ous slime to have an active role in their aboral poles 

 only; this is especially true for those polyps that have become 

 secondarily solitary animals, as is the case with Hydra. The gas 

 gland in the pneumatophore of numerous Siphonophora re- 

 presents a special case. It is proved by such isolated and special- 

 ized cases that mutations in the skin gland cells can reappear 

 again and again. 



In Cnidaria we see how the skin progressively loses the 

 character ot a mucous membrane, a fact which can be explained 

 as being due to the formation of cormi and, in connection with 

 this, to the sessility and to the formation of a periderm. In 

 Turbellaria, on the other hand, the character of the mucous 

 membrane has been better preserved; it has especially been 

 developed at the anterior ends (the deeply sunk-in front 

 glands) and on the ventral surface; here it has been specialized 

 and the slime has been given an important role which enables 

 the animal to creep over the substratum. This property has 

 later been inherited and further developed, by others (e.g. 

 Gastrotricha), and also by Mollusca. This slime secreted by 

 the skin glands can become so firm that it can be extended 

 into glutinous threads that can be used by the animal for 

 the formation of nets by means of which the animal can catch 

 its prey, and for the formation of thin cords which the animal 

 uses to descend. What else can the colloblasts of Ctenophora 

 be than specialized cells of skin glands which behave like 



