ALTERNATION OF GENERATIONS. 1 2.*> 



the umbrella by an investing sac, in which the canals are 

 imperfect or altogether wanting. 



Even in allied species, it is to be noticed, that there is a 

 great diversity in the development of these structures. 

 Thus in Laomedea geniculata the reproductive elements are 

 contained in free swimming metdusoids with marginal 

 cirrhi, and a central proboscidiform mouth, round the at- 

 tachment of which the ovarian chambers are situated. In 

 Laomedea Loveni,^ they assume the form of an inflated 

 bell surrounding the ovary, and fringed at its free edge with 

 minute tentacles ; and they remain during their brief period 

 of life attached to the edge of the horny " o vigorous cap- 

 sule" characteristic of the gToup, and there emit the sper- 

 matozoa or ova with which they were charged ; after which 

 they wither like blossoms, to be succeeded by a new expan- 

 sion. In Laomedea lacerata the ovarian sac advances to 

 the mouth of the capsule, but instead of a bell-shaped en- 

 velope becomes invested merely by a thick gelatinous coat. *(' 

 Even in the same species there is sometimes as great a 

 diversity in the opposite sexes ; thus in L. geniculata the 

 ova are formed in free swimming medusoids, the sperma- 

 tozoa in simple cysts, permanently attached. J 



Professor Allman refers to the reproductive sacs of 

 Tubular ia indimsa, which never become detached, as of 



* Allman in Annals of Nat. Hist., 3d Ser., IV., p. l-tO — tlie same 

 species apparently which Dr. Wright refers to as confounded by Johnston 

 with another having free medusoids, under the name of L. dichotoma . 

 (Edin. Philos. Journal, Jany, 1859, p. 110). This form of extracapsular 

 medusoid, Allman terms vieconidiuin, from its resemblance to a poppy- 

 head. 



t Dr. Wright, Op. Cit. 



X At least Professor Schultze is quoted by Dr. Wright (Edinburgh 

 Philosophical Journal, July, 1856, p. 85, note) as having discovered 

 sperm capsules in this zoophyte, but there is a degree of confusion among 

 the species which somewhat invalidates the conclusion iu the text. 



G 2 



