256 



COMPARATIVE ANATOMY 



CHAP. 



dermal) pericardial wall, which becomes separated from the pericardium, forms the 

 rudiment of the gonad, while an ingrowth from the base of the mantle cavity runs 

 towards this, and is the (ectodermal) rudiment of the genital .duct. The latter 

 arises on one side of the anus, just as the efferent duct of the permanent kidney 

 rises on its other side ; this ontogenetic fact confirms what was stated above (p. 219) 

 that the genital duct of the Monotocardia corresponds with a part of the right (which 

 originally, and in the young embryo, is the left) kidney of the Diotocardia (apparently 

 wanting in the Monotocardia}. 



The vascular system arises very early in the form of spaces between the mesoderm 

 and ectoderm or entoderm, round which the mesoderm cells grow, and which become 

 secondarily connected with the heart. 



All the ganglia of the nervous system, the cerebral, pleural, pedal, parietal, and 

 visceral ganglia arise separately as ectodermal thickenings, which become constricted 

 off from the ectoderm by delamination. They only secondarily become connected 

 with one another through the growing out of the nerve fibres. The parietal ganglia 

 arise to right and left in the middle region of the body, but soon become shifted by 

 the displacement of the organs of the visceral dome, one above the intestine and the 

 other below it. The rudiment of the visceral ganglion is said to appear dorsally to 

 the hind-gut and to move later to its position beneath the same. 



The observations on the development of Paludina vivipara, here briefly described, 

 are in many ways of great importance, and confirm in the most unmistakable 

 manner the results arrived at by comparative anatomy. The following are specially 

 noteworthy. 



1. The manner in which the pericardium originates favours the opinion that it 

 is a secondary body cavity. It is important to note that the pericardium is at 



first paired, being divided into 

 two lateral halves by a septum, 

 which afterwards disappears. 



2. The fact that the gonad 

 arises as an outgrowth of the 

 pericardium, confirms the view 

 arrived at by comparative 

 anatomy, that the genital cavity 

 also is a secondary body cavity. 



3. The anus and the mantle 

 cavity originally lie symmetri- 

 cally at the posterior end of 

 the body, but, through asym- 

 metrical growth, come to lie 

 first on the right side of the 

 visceral dome, and finally on 

 its anterior side. 



FIG. 212. Larva of Oncidium celticum, from the left The development of other 

 side (after Joyeux Laffuie). 1, Cerebral ganglion ; 2, edge Gastropods cannot here be de- 

 of the mantle; 3, rudiment of the gonad; 4, larval shell- scribed in dptail Wp rpfpr t>lp 

 muscle; 5, hiud-gut ; 6, rudiment of the digestive gland; C , 



7, auditory organ ; 8, pedal ganglion ; 9, foot ; 10, oesophagus ; rea(ler to the bibliography at 

 11, eye ; 12, branched muscle cells of the velum ; 13, velum, the end. As a rule, nutritive 



yolk is present in larger 



quantities than in the viviparous Paludina, in which the small provision of 

 yolk is evidently connected with the favourable conditions of nutrition of the 

 embryo. 



The blastopore generally corresponds in position with the future mouth ; it often, 



