180 LECTURE IX. 



medusae are still younger, the organ is only a flat band, without in- 

 equalities. Each testis consists of a band with many folds ; the whole 

 bent in a bow, with the convexity attached to the concave wall which 

 divides the generative cavity from the stomach. 



If a probe be inserted into the generative cavity, it immediately 

 touches the under or outer surface of the testis; if it be passed into the 

 digestive cavities, it touches the upper or inner surface of the testis, 

 but not immediately, the epithelium of the digestive cavity covering 

 this surface of the testis. The testis is much longer than the cavity 

 containing it, but is adapted thereto by its numerous folds. Its con- 

 cave side gives off a numerous series of highly irritable coloured 

 tentacles, having the same structure as those on the arms : they are 

 richly ciliated, and contain many hyaline rounded corpuscles and 

 thread-cells immediately beneath the surface. The spermatic ten- 

 tacles are capable of only moderate extension : at the breeding time 

 they pi'oject from the mouth of the generative cavity, leaving only 

 a small passage in their centre. By their powerful ciliary apparatus 

 they keep up a strong current of sea-water in the cavity, and thus 

 aid in the expulsion of the semen. No ciliary movement is observable 

 upon the delicate epithelium covering the lower surface of the testis. 

 The parenchyma of the testis consists of a transparent granular sub- 

 stance in which are imbedded innumerable pyriform sacs, having their 

 basis turned towards the upper surface of the testis, and their apical 

 orifices opening upon the under-surface of the testis, which they 

 make uneven by their tumid margins. The spermatozoa are developed 

 in these sacs, which permanently represent the eai'liest rudiments 

 of the extremely elongated seminal tubes in higher animals. The 

 parietes of the seminal sacs are pretty thick, and, perhaps, contractile. 



In young males the sperm-sacs contain numerous cells with many 

 nuclei, from each of which nuclei a spermatozoon is developed ; and 

 as this development proceeds, the sperm-cell presents a striated cha- 

 racter, and, lastly, a fasciculus of spermatozoa. These have an enlarged 

 end, or body, and a filiform appendage, so fine as scarcely to be seen 

 save by its undulatory movements. The fasciculus of spermatozoa 

 does not exhibit these parts in the same degree of development in each 

 sperm-sac : those nearest the cervix of the sac are the most perfect. 

 The tails of the spermatozoa are always directed towards the opening 

 of the sperm-sac. The bundles of these filaments follow each otiier, 

 and often the apical tails of one bundle are infixed in the central 

 interspace of the bodies of the preceding bundle, and a chain or string 

 of bundles are thus formed which are easily seen by a moderate 

 microscopic power. These spermatozoa are very lively in sea-water. 

 In order to observe the actual ejaculatio seminis, cut off a piece of the 



