770 THE ANIMAL KINGDOM. 



The Siphonophora, the third order of Craspedota, are pelagic and 

 colonial. The various parts which may enter into the composition of 

 a colony 1 are as follows, (i) The polypite or gastrozooid, universally 

 present, usually attached to the coenosarc of the colony by a longer or 

 shorter pedicle, and consisting of three regions, a basal with thickened 

 ciliated ectoderm containing cnidoblasts, a central somewhat dilated, and 

 a terminal oral ciliated portion, extremely changeable in shape. Cnido- 

 blasts are to be found round the mouth. The endoderm cells are vacuo- 

 late, and those of the middle region are pigmented, produced into longi- 

 tudinal ridges 2 or villiform processes. (2) Hydrocysts or feelers ( = Taster 

 of German writers), absent in Calycophoridae and Discoideae. These struc- 

 tures are polypites in which the distal or oral extremity is imperforate and 

 usually armed with cnidoblasts 3 . The pedicle is absent or short, and the 

 three regions (supra) are not differentiated, no trace of the basal ectodermic 

 thickening being even discernible. The endoderm is vacuolate and rarely 

 elevated into ridges (Apolemia}. The hydrocysts are represented in the 

 Discoideae by small zooids (blastostyles) with mouths. (3) Tentacles. 

 These structures in the Discoideae are simple and tubular ; in Porpita 

 dilated terminally and provided with a number of short capitate processes. 

 The large tentacles of Physalia attain sometimes the length of many feet. 

 Each tentacle, whether large or small, consists of a conical hollow sac (? = 

 hydrocyst) covered with cnidoblasts, a long hollow filament attached to the 

 base of the sac but connected by a membranous expansion with its side. 

 The first section of the filament is coiled from side to side, but the greater 

 portion of it hangs freely. One of its aspects is covered with transverse 

 hollow reniform elevations in which cnidoblasts are aggregated. The 

 tentacles of other Siphonophora are, with the partial exception of Athorybia 

 and Abyla, attached to the pedicles of the polypites, on a special elevation 

 of ciliated ectoderm containing cnidoblasts, from which new ' nettle bat- 

 teries' are derived in growth. They are branched except in Apolemia, and 

 each branch terminates in a single nettle battery, the structure of which is 

 often extremely complicated and characteristic of a genus, or even species. 

 The tentacles of the hydrocysts are unbranched. (4) The sexual zooids or 

 gonozooids are medusiform. In the Discoideae they are medusae, and are 

 borne in numbers upon blastostyles (gonoblastidia) which are disposed in a 



1 See p. 775, post, on the character of the colony. 



2 They are due to the contraction of endodermal circular muscles. 



3 An opening is present at the apex of the hydrocyst of Halistemma tergestinum (pictuni) and 

 of Physalia (?). So too in a young Agalmopsis (Agalma] Sarsii, where it afterwards closes. The 

 apex of the hydrocyst is pigmented, and it has been observed that irritation of the animal in Forskalia 

 causes this pigment to be shed into the water, giving the latter an opaque and red tint. The con- 

 tained fluid is highly albuminous and dense, and probably serves to keep the walls of the hydrocyst 

 tense, and thus render it more sensitive (Korotneff). Hydrocysts occur among the nectocalyces in 

 Apolemia but not in other Siphonophora. 



