SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 97 



stimulated directly by environmental changes ; in all other animals 

 there is intimately associated with the contractile muscle tissue a 

 specifically receptive and conductive nerve tissue, through which 

 environmental stimuli become effective on the muscles. Finally, the 

 high metabolic rate of muscular activity creates the need of special 

 excretory organs for the removal of waste products from the lx)dy. 

 The genesis of contractile and conductive tissues, and their inte- 

 gration into a neuromuscular system are best seen in the Coelenterata. 

 Contractility, being a common property of protoplasm, may become 

 localized and specially developed in a particular part of any cell of 

 the body in a primitive animal. In the coelenterates fingerlike muscle 

 processes are produced from the inner ends of cells in both the 

 ectodermal and the endodermal epithelium, those of the ectoderm 

 (fig. 3, iiip) taking a longitudinal course, those of the endoderm a 



CnL 



SCI 



Ecd<: 



SL— ' 



Fig. 3. — Diagram of the ectodermal neural and muscular elements of Hydra. 

 (From Curtis and Guthrie, 1927.) 



Cnb, cnidoblast ; Ecd, ectoderm ; ntf, muscle fiber ; mp, muscle process of 

 epithelial cell ; NCI, neural cell ; NSCl, neurosensory cell ; SCI, sensory cell ; 

 SL, supporting lamella. 



transversely circular course. Fibrils of contractile tissue (nif) be- 

 come differentiated in these processes. In the hydra, the body of the 

 muscle cell remains as a part of the epithelial layer, but in some of 

 the other coelenterates the entire cell may be withdrawn beneath the 

 surface and converted into a muscle fiber. A primitive nerve cell is 

 an epithelial cell in which the common protoplasmic properties of 

 irritability and conductivity are specially developed both in the cell 

 body and in branching processes given off from the latter, but the 

 nerve cells become differentiated into superficial receptive cells and 

 deeper-lying conductive cells. In the hydra the receptive cells (fig. 3, 

 SCI) and the sensory cells (SNCl) contained in the ectoderm have 

 connections, on the one hand, with the surface of the body, and, on 

 the other, send branches to the strictly neural cells (NCI), which are 

 distributed through the inner parts of the ectoderm, and in turn 

 send branches to the muscle processes of the muscle cells. The endo- 



