INVERTEBRATE ANIMALS. 



CHAPTER VII. 



SUB-CLASS SIPHONOPHOEA. 



The animals included under the name of Siphonophora are often 

 known as the " Oceanic Hydrozoa,'' as they are not fixed like the 

 Hydroid Zoophytes, but are found swimming at the surface of the 

 open ocean, far from land. They are all singularly delicate and 

 beautiful organisms, but they require little notice here. They are 

 distinguished from the Hydroid Zoophytes, which we have been just 

 considering, by the fact that the hydrosoma consists of numerous 

 polypites, united by a common trunk or coenosarc, which is very 

 rarely branched, and is never furnished with any hard outer cover- 

 ing or polypary, so that it remains permanently soft and flexible 

 throughout life. The proximal end of the coenosarc, as already re- 

 marked, is not fixed, or capable of being fixed, to any solid object, 

 but is variously modified to suit the requirements of the floating 

 colony. As in the Hydroida, the reproductive organs are in the 

 form of special buds, which have the power of developing the essen- 

 tial elements of generation, and which are often detached as free- 

 swimming medusoids. 



The entire sub-class is divided into two great groups or orders, 

 and it will be sufficient to consider shortly a typical form of each. 

 In the first order — that of the Calycophorida! — the ccenosarc is thread- 

 like, cylindiical, unbranclied, and highly contractile. The cavity of 

 the ccenosarc dilates proximally into a peculiar ciliated chamber, 

 which is the distinguishing character of the oi'der. The name of 

 Calycoplioridce (Gr. ktihix, a cup ; and phero, I bear), is, however, 

 derived from another circumstance — namely, that the proximal end 

 of the coenosarc is always furnished with a series of bell-shaped discs, 

 which are known as " swimming-bells " or " nectocaly(?es." Each 

 nectocalyx consists of a bell-shaped cup (fig. 39, n), attached by its 

 base to the ccenosarc, and having its cavity turned outwards. In 

 the substance of the disc run at least four canals, which communi- 

 cate with the cavity of the coenosarc, and proceed to the margin of 

 the bell, where they all open into a circular vessel. The mouth of 



