304 . Heredity. 



throiigli a fixed poljp-like Jarval stage before maturity 

 is reached, and this polyp hirva is destitute of a swim- 

 ming umbrella, and of organs of special sense. It has 

 an elongated cylindrical body, by one end of which it is 

 attached, while the mouth is jolaced at the ojiposite end 

 and is surrounded by a crown of tentacles. 



The group is divided into two grand divisions, the 

 medusae wdth a veil or diaphragm across the ojiening of 

 the umbrella, and the medusae without a veil. The two 

 groups resemble each other in all essential particulars, 

 and no naturalist has doubted that they are truly homol- 

 ogous with each other, but they present certain constant 

 differences, such as the presence of a veil and the ab- 

 sence of gastric filaments in the one group, and the ab- 

 sence of a veil and the presence of gastric filaments in 

 the second group. The larva of a veiled medusae is a 

 hydroid-polyp, which has a simple digestive cavity, and 

 the power of multiplication by lateral budding, while 

 the polyp-larva in the veilless medusae is known as a 

 gcyphistoma. It has gastric filaments in its digestive 

 cavity, and it multiplifs by terminal budding or fission. 

 In other respects, the two kinds of larvae show a close 

 homology with each other, but the points of resemblance 

 are not the same as those w^hich unite the two groups 

 of mature medusae. 



Haeckel has devoted many years to the study of the 

 medusae, and his opinion is entitled to very great 

 weight, and he believes that the resemblances between 

 the larvae are due to community of descent, but that 

 the resemblances between the adults are not. lie be- 

 lieves that the remote ancestor of all the m.edusae was a 

 polyp which united in itself the features which now dis- 

 tinguish the hydra-larvae of the veiled medusae from the 

 scyphostoma larvae of the veilless forms, and that these 



