CCELENTERATA : SIPHONOPHORA. 135 



man-of-war (Physalia} the pneumatocyst communicates with 

 the exterior by means of an aperture in the ectoderm of the 

 pneumatophore. In Velella and Porpita the pneumatocyst 

 communicates with the exterior by means of several similar 

 openings called "stigmata;" and from its distal surface de- 

 pend numerous slender processes containing air, and known 

 as " pneumatic filaments." 



The polypites of the Physophorida resemble those of the 

 Caly cop hor idee in shape, but the tentacles have a much more 

 complicated structure, and are sometimes many inches in 

 length, as in Physalia. The " hydrophyllia " have essentially 

 the same structure as those of the former order. There occur 

 also in the Physophorida certain peculiar bodies, termed 

 " hydrocysts " or " feelers " ( fiihler " and " taster " of the Ger- 

 mans). These resemble immature polypites in shape, consist- 

 ing of a prolongation of both ectoderm and endoderm, usually 

 with a tentacle, and containing a diverticulum of the somatic 

 cavity, the distal extremity being closed, and furnished with 

 numerous large thread-cells. They are looked upon as "organs 

 of prehension and touch," and they are somewhat analogous 

 to the " nematophores " of the Plumularians. 



As regards the reproductive organs, they are developed 

 upon special processes or "gonoblastidia," and they may re- 

 main permanently attached, or they may be thrown off as 

 free-swimming medusoids. In many of the Physophorida the 

 male and female gonophores differ from one another in form 

 and size, and they are then termed respectively "andro- 

 phores" and " gynophores." As regards their development, 

 the Physophoridce obey the same general law as the Calyco- 

 phoridcz. 



In Physophora the hydrosoma consists of a filiform coenosarc, which 

 bears the polypites and their appendages, and dilates proximally into a 

 pneumatophore. Below this point the coenosarc bears a double row of 

 nectocalyces, which are channelled on their inner faces to allow of their 

 attachment to the coenosarc. There are no hydrophyllia, but there is a 

 series of "hydrocysts" on the proximal side of the polypites. 



Physalia, or the Portuguese man-of-war (fig. 56, a) is composed of a 

 large, bladder-like, fusiform "float" or pneumatophore sometimes from 

 eight to nine inches in length upon the under surface of which are arranged 

 a number of polypites, together with highly contractile tentacles of great 

 length, " hydrocysts," and reproductive organs. Physalia is of common 

 occurrence, floating at the surface of tropical seas ; and fleets of it are not 

 uncommonly driven upon our own shores. 



In Velella (fig. 56, b) the hydrosoma consists of a widely-expanded pneu- 

 matophore of a rhomboidal shape, carrying upon its upper surface a dia- 

 gonal vertical crest. Both the horizontal disc and the vertical crest are 

 composed of a soft marginal "limb," and a central more consistent "firm 

 part." "To the distal surface of the firm part of the disc are attached the 



