TYPE CCELENTERA. 97 



occurs in one species of Millepora having its reproductive organs in the 

 walls of the manubrium that the affinities of the group are with the Antho- 

 medusae, and that an exceptional amount of degeneration of the medusa 

 had occurred in correspondence with the development of the calcareous 

 corallum. 



The Siphonophores are evidently allied to the Anthomedusae, judging 

 from the characters of their medusae. The colony, however, contains both 

 medusa and polyp individuals, the former not being in all cases reproduc- 

 tive as in the Anthomedusae. The embryology of those members of the 

 order which have been studied in this particular indicates that they too must 

 be regarded as produced by budding in embryonic stages, some of the buds 

 developing to medusae, others remaining in the polyp stage. A further 

 differentiation of the medusae took place by which the pneumatophore, 

 nectocalyces, and covering scales have been specialized from medusae origi- 

 nally reproductive, the pneumatophore probably representing the parent 

 individual of the colony. It is interesting to note in this connection that 

 in some forms the reproductive rnedusaa after having expelled their ova or 

 spermatozoa become converted into nectocalyces. 



II. CLASS SCYPHOMEDUSJE. 



Iii the Scyphomedusse the medusa form is preeminent, the 

 polyp form being placed in the background and occurring 

 only as a larval stage, though in some forms it assumes 

 somewhat greater importance on account of the power it may 

 possess of reproduction by transverse division. 



The medusae are usually free-swimming, though a few of 

 the more lowly organized forms are attached throughout their 

 lives by a prolongation of the exunibrellar surface (Fig. 52), 

 forming a connecting link between the free-swimming forms 

 and the polyp. As a rule they reach a much greater size 

 than do the Hydromedusae, from which they are further dis- 

 tinguished by (a) the absence of a velum, (6) by the sense- 

 organs when present being modified tentacles, and (c) by the 

 reproductive cells always arising in the endoderm. On ac- 

 count of the first of these characters the Scyphomedusse are 

 sometimes termed the Acraspeda. 



They are all more or less bell-shaped, a number of ten- 

 tacles usually hanging from the margin of the bell (Fig. 53). 

 These, however, are frequently secondarily developed, there 

 being in the simpler forms eight primary tentacles which may 

 persist as tentacles, or four or all of them may be converted 



