80 COMPARATIVE AN'ATOMY. 



§ <33. 



In tlie cortical layer of the body of tlie Gregariu^e, and of many 

 Infusoria^ tliere are indications of bands, or fibres, resembling- 

 muscles. In the Gregarinee these structures are arranged cir- 

 cularly, or spirally, and form a layer just below the cuticle; it 

 extends over a short part only of the " head,^^ which, as a rule, is 

 separated from the body by a constriction, but it never passes into 

 the wall of partition which separates this part from the body. 



Among the Infusoria these contractile bands are principally 

 known in the larger species of some genera (Spirostomum, Stentor, 

 Prorodon, etc.). In others they are absent. They sometimes run 

 spirally, sometimes longitudinally. They are also present in the 

 Vorticellinfe, where they form spiral coils towards that end of the 

 body which passes into the stalk. It is clear that these structures 

 do not form the sole contractile system of the body, for those 

 Infusoria which do not possess them are ca]3able of executing 

 powerful contractions. But that they are really contractile is shown 

 by Spirostomum, in which the contractions of the body are not 

 effected along its long axis but in the direction of the striated band, 

 which describes several spiral turns. The contractile band, which 

 runs in the interior of the stalk of the Vorticellinte, is a differ- 

 entiation from the protoplasm of the same kind ; in Zoothammium it 

 branches with the colony, in Carchesium it exists separately in 

 each individual of the colony. Although these structures in many 

 points resemble muscular fibres, and are physiologically the same, 

 they cannot be compared with these histological elements from 

 a morphological point of view, for neither cells nor the products 

 of cells take any share in forming them. They are differentiations 

 from the protoplasm of the organism, whilst in the tissues of the 

 Metazoa they are formed by the differentiation of a whole lot of 

 morphological elements. The whole contractile apparatus cor- 

 I'esponds therefore to a muscular system in function only. The 

 separate bands or stripes are merely analogous to muscles 

 (myophana) . 



§ 64. 



The function of supporting organs of the body is performed 

 in the Protozoa by firm structures, which either traverse the soft 

 substance of the body as a framework, or invest it in the form of 

 shells and tests. The latter serve as organs of defence in proportion 

 to their size and firmness. All the structures here to be enumerated 

 are either directly or indii-ectly differentiations of the protoplasm, 

 which are formed either on the surface or in the parenchyma of the 

 body. The more completely these secretions cover the body in the 

 form of tests, the more do they interfere with its freedom of 

 movement, unless there are compensating aiTangements. Shells and 

 internal supports are widely distributed in all divisions of the lower 



