108 COMPARATIVE ANATOMY. 



Muscular System. 

 § 83. 



Form-elements, referable to muscles, are not certainly known to 

 exist among the Spongiae ; in the more exactly known Calcispongke 

 they are certainly absent, and the protoplasm of the ecto- and 

 endoderm performs all the movements of the animal. 



A muscular layer is first marked off in the Hydromedusas 

 (Hydriformes), where the cells of the ectoderm possess band-like 

 processes, which form a connected stratum beneath that layer of 

 cells (cf. supra, § 25). This layer is also continued on to the ten- 

 tacles, but is wanting in those parts of the colony which are sur- 

 rounded by a test. In some parts, as in the stem of the colony in 

 the Siphonophora, it is greatly developed. In the Medusas it is 

 limited to the surface which carries the gastric apparatus, where it 

 forms the " sub-umbrella." From the edge of the bell or disc it 

 passes into a more or less broad membranous process, the velum, 

 which consists essentially of muscular fibres ; it is also continued on 

 to the tentacular organs. The muscular system is more complicated 

 in the Discophora, many of which also are provided with a velum 

 (Aurelia). In all the Medusas the form-elements of the muscles 

 are finely striped transversely, but this is not the case in the same 

 parts of the Hydriformes. 



In the Ctenophora muscular bands have been observed running 

 along the ciliated " ctenophores ;" and there are muscular fibres in 

 the interior of the gelatinous tissue of the body. 



The muscular system appears to be best developed in the 

 Anthozoa. Thus, in the Actinias, the attached base of the body is 

 distinctly formed by muscles, and circular and longitudinal fibrous 

 lavers can be made out on the rest of the body, which are continued 

 into the tentacles. In those Anthozoa that form stocks, the bodies 

 of the separate animals appear to have circular and longitudinal 

 muscles, whilst the soft ccenenchyma is also contractile, the network 

 of canals of the gastrovascular system, which traverses it, being 

 accompanied by muscular fibres. 



Nervous System. 

 § 84. 



The Spongias are placed in the lowest grade of animal differentia- 

 tion, owing to the absence in them of any arrangements which can 

 be regarded as special organs of sensation. The Acalephas are 

 not far above them, for their lower forms likewise give no signs 

 of distinct organs of this kind. Thus the cellular layer of the 



