40 THE HYDROMEDUSAE 



modifications of polymorphism carried in this group, that it is 

 sometimes impossible to form an opinion as to whether a particular 

 structure is to be regarded as tentacle or dactylozooid. 



3. The blastostyle (gonostyle, sexual palpon or siphon) which 

 produces sexual medusoids by gemmation, is generally devoid of 

 a mouth, but not invariably (Velella Fig. 48, EL), It develops 

 no tentacles. The blastostyle sometimes branches into a gonodendron 

 (Physalia), (Fig. 51). 



POLYMORPHIC MODIFICATIONS OF THE MEDUSOID : 



4. The sexual medusoid is set free from the colony ($ Physalia, 

 Velella) or remains fixed ( 6 Physalia). It may have the typical 

 structure of the Anthomedusan medusoid (Velella), or may exhibit 

 the various stages of arrest in development already described (p. 

 20). Even when arrested at an early stage it is sometimes freed, 

 and swims by means of cilia ( $ Forskalea) ; more often it is a 

 permanently fixed sporosac ( 6 Physalia). The medusoid is budded 

 from a blastostyle (Velella, Fig. 48), from the coenosarc (Agalmopsis), 

 or from the pedicle of the gastrozooid (Diphyes, Fig. 47). Most 

 colonies of Siphonophora are hermaphrodite, and in some cases 

 so also are the gonodendra (Physalia) ; the medusoids are either 

 male or female. 



5. The nectophore (nectocalyx, nectozooid) is a medusoid 

 devoid of tentacles, manubrium, and mouth, but retaining the 

 characteristic velum, circular, and radial canals (Figs. 44, k; 45, 

 m ; 46, N'N"). The musculature is well developed. The necto- 

 phore has a locomotor function. 



6. The hydrophyllium (bract, phyllozooid) is a shield-shaped 

 medusoid, of protective function (Figs. 44, I ; 47, H). It consists 

 typically of a somewhat curved plate of thick mesogloea, covered 

 externally by ectoderm, and containing a solid endodermal core 

 (phyllocyst). Its medusoid origin may be inferred from a few 

 species in which it retains a structure intermediate between that of 

 medusoid and typical hydrophyllium ; in Athoria, for example, 

 its apex is excavated into a rudimentary subumbral cavity with 

 minute circular and radial canals, and four knobs representing 

 rudimentary tentacles. 



7. The pneumatophore, an apical air sac of hydrostatic function, 

 appears under two quite distinct forms. 



(a) In the Physonectae and Cystonectae, it is probably a highly 

 specialised medusoid, the exact homologies of which are obscure 

 (Figs. 44, n ; 45, a' ; 49, PN). It is formed typically as a swelling 

 at the upper end of the coenosarc, into which in the course of 

 development an entocodon pushes its way. The ectodermal 

 cavity thus produced is distinguishable into two regions a 

 central part, the air gland, secretes a gas which passes through 

 a pylorus into a distal part, the air sac, lined by a chitinous 



