MUSCULAR AND ALLIED SYSTEMS. 91 



vert, which originate from the longitudinal layer on a level 

 with the anus and are inserted at the base of the tentacles. 

 The individual muscle-fibres consist of an outer sarcolemma, 

 a spirally fibrillated contractile cortex and a central proto- 

 plasmic portion. Upon the right, several of the longi- 

 tudinal bundles have been removed to show the underlying 

 diagonal layer, and on the left the circular layer is similarly 

 exposed by the removal of both longitudinal and diagonal 

 layers. 0. C. 56 A. 



Andreae, Zeitschr. f. wiss. Zool., Bd. xxxvi. 1882, p. 228. 



C. 15. A similar preparation of Phascolosoma vulgare. In this 

 Gephyrean the muscles of the body-wall are far more 

 delicate than in Sipunculus and are arranged in two layers 

 only longitudinal and circular in both of which the fibres 

 form a continuous sheet. The course of the fibres in the 

 two layers can be seen where black paper has been inserted 

 beneath them. 0. C. 56 B. 



C. 16. Right half of a Sea-Mouse (Aphrodite aculeata) showing 

 the muscular system, which in this worm attains a very 

 high degree of complexity. The muscle-layers of the body- 

 wall are broken up into a number of individual muscles, the 

 most conspicuous of which are six longitudinal bands a 

 pair along the dorsum contiguous with each other, a ventral 

 pair separated by an interval of 7 mm., and a single band 

 along each side. The dorsal and ventral bands are contin- 

 uous from end to end of the body, but the lateral bands are 

 interrupted and firmly attached to the body- wall between 

 each segment by a tendinous intersection. The muscles of the 

 outer (circular) layer form a network of diagonal and trans- 

 verse bands that pass between and superficial to the longi- 

 tudinal bands. The interstices between them are occupied 

 by the hepatic caeca. A large series of perpendicular and 

 diagonal muscles traverse the body-cavity between each 

 segment, and each parapodium has its own particular pro- 

 trusor and retractor muscles. 0. C. 57 A a. 



C. 17. Portion of a Nereis {Nereis foliosa) showing two series 

 of muscles, which extend along the dorsal aspect from one 



