i ORGANISATION AND STRUCTURE OF MUSCLE 11 



position, they unite into groups, the periphery of which is formed 

 of the contractile fibrils, while the axis contains the corresponding 

 nuclei and protoplasm. 



Between this arrangement and the original superficial dis- 

 position of the muscle-fibrils there are numerous transitional 

 stages, produced by involution of the muscular lamella, which 

 obviously tends to increase the mass of the muscle with stationary 

 body-superficies. As long as the folding of the muscle-lamella is 

 not excessive, the irregularities which it produces towards the 

 free surface are equalised by the varying lengths of the epithelial 

 cells. The supporting mesenchyme also presses from 

 within into every fold. The involution varies con- 

 siderably in degree. Sometimes " muscle-plates " are 

 produced which stand perpendicular to the body- 

 surface in close juxtaposition, like the leaves of a 

 book (Fig. 3). Each leaf consists of a thin support- 

 ing lamella of mesenchyme, set on both sides with 

 muscle-cells. It is easy to see how by such a 

 process of involution and segregation, carried one step 

 farther, the cylindrical fasciculi of muscles, entirely 

 surrounded by mesenchyme, may be developed. 



In Medusa we meet with conditions similar to ^ j 

 those exhibited by Actinia. The muscle-fibrils, which FIG. 3. Trans- 

 are often transversely striated, everywhere exhibit a ofthemSies 

 basal differentiation of ectodermal, epithelial cells, of the body. 

 which again serve in many cases to bound the body- J^Lf mem- 



Surface. Iranaceus. 



A structure and development of muscle, similar in 

 many respects to that already described in Cnidaria, exists conspicu- 

 ously among many Worms (Annulata), where the epithelial, or at 

 least epithelioid, character of the muscle is still immediately recognis- 

 able in the simplest cases. Here the longitudinal muscle-fibres 

 often consist of mononuclear, elongated cells, arranged like a single- 

 layered epithelium. Each muscle-cell isolated, or in transverse 

 section shows two distinctly separate substances, an internal 

 plasmatic portion, and an external contractile substance, which 

 again is constructed of countless smooth fibrils, running parallel 

 with the long axis of the cell, and, as seen in cross- section, arranged 

 in lamin?e lying close together, so that the contractile layer 

 exhibits a delicate radial striation. Each single stripe corresponds 



