262 



COMPARATIVE ANATOMY 



CHAP. 



the ectoderm of the cephalic cavity, in the foot and on the intestine (especially on 

 the oesophagus, where they are already changed into muscle cells) ; and (2) two 

 mesoderm streaks lying at the sides of the intestine. The pedal ganglia arise 

 together with the paired rudiment of the byssus gland, as thickenings of the 

 ectoderm at the posterior end of the foot. The auditory vesicles originate as 

 ingrowths of the ectoderm. The mantle forms by degrees from behind forward as a 

 ridge, which grows more and more ventrally downwards. At the same time the 



FIG. 218. A-D, Four stages in the development of Cyclas cornea, from the right side (after 

 Ziegler). 1, Membranous shell ; 2, rectum ; 3, anus ; 4, free edge of the mantle ridge or fold ; 

 5, rudimentary byssus cavity with gland ; 0, rudiment of the pedal ganglion ; 7, foot ; 8, velar 

 region; 9, oesophagus ; 10, stomach; 11, calcareous shell; 12, pericardium; 13, kidney; 14, rudi- 

 ment of the gonad ; 15, edge of the membranous shell ; 16, edge of the calcareous shell ; 17, rudi- 

 ment of the gill ; 18, byssus thread ; 19, visceral ganglion ; 20, posterior adductor ; 21, glandular 

 part of the kidney ; 22, lateral wall of the pericardial vesicle ; 24, median wall of the same ; 

 25, digestive gland (liver) ; 26, cerebral ganglion ; 27, mouth ; 28, auditory vesicle. 



shell gland, which at its edge secretes the delicate shell membrane, spreads out and 

 becomes flattened. Beneath the shell-membrane the rudiments of the permanent 

 shell valves are produced from two small round areas lying to the right and left of 

 the dorsal middle line (B). The digestive gland (liver) develops from two lateral 

 globular outgrowths of the wall of the stomach. The gonads arise from cells of the 

 mesoderm streaks, which are larger than the rest and also in other ways differen- 

 tiated, so that they can very early be distinguished. In the anterior and dorsal 



