344 ANIMAL BIOLOGY. [Part II. 



(sub-umbrella), and the surface of the manubrium, is covered 

 with an epithelial layer, beneath which is a thin sheet of 

 muscular tissue, not thicker than very thin paper. And between 

 these two there is an exceedingly delicate plexus of nervous 

 tissue, containing both nerve-fibres and ganglion cells. Beneath 

 the epithelium of the outer surface of the nectocalyx there is 

 neither muscular nor nervous layer. In the velum there are 

 concentric muscular fibres, but probably no nervous tissue. 



The nectocalyx is rhythmically contractile. Hence the 

 medusa can swim lazily through the water. But if the margin 

 containing the nerve-ring be cut off, immediate, total, and per- 

 manent paralysis of the umbrella results. 



It should be noted that this medusa or free gonophore is in 

 several respects more highly organised than the hydroid stock 

 from which it takes its origin. It has (1) between the endoderm 

 and ectoderm a thick mass of intermediate substance; it has (2) 

 a well-defined nervous system ; and it has (3) sense organs. 



The segmentation of the ovum is complete or holoblastic. A 

 morula results, which passes into a single-layered, ciliated, 

 planula. The segmentation cavity becomes the gastric cavity of 

 the future polyp, but becomes lined with a second layer of cells 

 derived from the first by delamination. The spherical or oval 

 larva, now free, after a short locomotive existence, fixes itself 

 by its aboral pole. At the opposite end a mouth appears; 

 tentacles bud out ; fresh individuals are produced by a process 

 of budding ; these, however, do not separate as in hydra viridis, 

 but remain attached, and form part of the colonial stock. 



Such, then, is the typical mode of reproduction in the hydro- 

 zoa, the sexual products ripen, and are fertilised in a separate 

 medusoid gonophore. In some cases, however, the medusa 

 never separates to lead a free existence; it is an attached medusa. 

 In others, the attached gonophore is incomplete; its essential 

 parts are present, but in an unexpanded condition; it is a 

 disguised medusa. In yet others development may go no further 

 than the production of a hollow diverticulum of the body-wall 

 of the parent ; it is a sporosac. The free medusa in the course 

 of its development passes in succession through the stages of 



