94 PHYSIOLOGICAL SERIES. 



C. 23. The sheath of an Acorn-shell (Balanus porcatus), laid open 



to show the attachments of the muscles to the moveable 



opercular valves. 0. C. 54. Hunterian. 



C. Darwin, Monograph of the Cirripedia, Ray Soc. 1854, 



p. 61. 



C. 24. Longitudinal section of a Lobster (Homarus vulgaris), 

 showing the muscles attached to the right half of the body. 

 The chief muscles are indicated by numbers as follows. 



I. Adductor of the eye-stalk. 2. Anterior gastric. 3. 

 Levator of antenna. 4. Depressor of antenna. 5. Sternal 

 levator. 6. Adductor of mandible. 7. Posterior gastric. 

 8. Tergo-epimeral. 9. Flexor of coxopodite of chela 

 (acting as an adductor of the whole leg). 10. Extensor 

 of basipodite of chela (acting as depressor of the leg). 



II. Intersternal. 12. Deep abdominal extensor. 13. Ab- 

 dominal rotator. 14. Deep abdominal flexor (enveloping 

 portion). 14*. Its transverse portion. The most note- 

 worthy of this highly specialised series of muscles are 

 those connected with the movements of the abdomen. 

 They consist of extensors (12), flexors (14), and rotators 

 (13). Both flexors and extensors are composed of super- 

 ficial and deep layers, the former in each case being very 

 poorly developed. The deep flexor is a remarkable muscle 

 both on account of its size and its complicated formation. 

 It rises on either side from abdominal and cephalothoracic 

 regions and extends to the telson, with attachments to 

 each abdominal segment. Broadly speaking, it is com- 

 posed of a central core of fibres around which an enveloping 

 sheet (14) is coiled in a spiral. In each segment offshoots 

 (14*) from the central core run transversely across the 

 body to the central core of the muscle of the other side. 

 The enveloping fibres with additions from the central 

 core are attached to the anterior margin of each abdominal 

 sternite, but the main part of the central core is attached to 

 each tergum above the hinge-line, so that the action of 

 the muscle is partly one of extension although mainly 

 of flexion. 



Owing to the immobility of the cephalothorax, the 



