288 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Tapeworms with hooked heads are found in carnivorous 

 mammals and in birds, where the cavity of the in- 

 testine is comparatively limited, but they have not 

 yet been seen in such Cestoda as live in herbivorous 

 mammals, where the intestinal tract is much more 

 spacious. The group term Acanthocephali, and the 

 generic name Echinorhynchus, refer to the presence of a 

 number of hooks on the " proboscis " of other parasites. 

 In the higher Nemertinea stylets are developed 

 at the base of the proboscis, and it is particularly 

 interesting to observe that, where not present, their 

 place is taken by stinging cells ; a similar correlation 

 is found among the Turbellaria, where the absence of 

 nematocysts is often atoned for by the presence of 

 small, rod-like structures, the so-called rtiabdites. 

 In the Chsetopoda some of the gland-cells of the 

 integument secrete hard chitinous bristles or setae of 

 various lengths, which are protective and locomotor 

 organs ; in the Oligocltseta (e.g, Lumbricus, the 

 earthworm) these setee are few in number, and never 

 exceed, so far as is known, eight in all ; in the marine 

 Polychseta they may be more numerous and much 

 larger than in the earthworm ; they may be variously 

 denticulated or hooked at their free ends, and may, 

 in the tube dwellers, aid the animal in raising itself 

 up its tube. 



The Polyzoa are provided with an organ of pro- 

 tection, which is in all cases external or of tegumentary 

 origin ; it may be soft and gelatinous, or harder and 

 chitinous, or calcareous. It has been, somewhat 

 unfortunately, called a cell ; it invests only the hinder 

 part of the body, but it may serve, in times of danger, 

 as a refuge for the more anterior portion, which can 

 be withdrawn into it. 



All Ecliinoderms, with the exception of the 

 Holothuroidea, have a well-developed skeleton, and 

 such is found also in some Holotliurians. It is formed 



