302 COELENTERATA HYDROZOA CHAP. 



in length of the stolon is accompanied by a complication of 

 structure, the simple tubular condition being replaced by a 

 spongy complex of tubes covered by a common sheath of 

 ectoderm. In the Auronectidae the stolon is represented by a 

 conical or hemispherical spongy mass bearing the zooids, and in 

 the Ehizophysaliidae and Chondrophoridae it becomes a disc or 

 ribbon- shaped pad spreading over the under side of the 

 pneumatophore. 



Gonozooids. — The gonozooids are simple tubular processes 

 attached to the stolon which bear the Medusae or the degenerate 

 medusiform gonophores. In the Chondrophoridae the gonozooids 

 possess a mouth, but in most Siphonophora they have neither 

 mouth nor tentacles. In some cases, such ■ as Anthophysa, the 

 colonies are bisexual — the male and female gonophores being 

 borne by separate gonozooids — but in others (e.g. Pliysalia) the 

 colonies appear to be unisexual. 



As a general rule the gonophores of Siphonophora do not' 

 escape from the parent colony as free-swimming Medusae, but an 

 exception occurs in Velella, which produces a number of small 

 free-swimming Medusae formerly described by Gegenbaur under 

 the generic name Chrysomitra. This Medusa has a velum, a 

 single tentacle, eight to sixteen radial canals, and it bears the 

 gonads on the short manubrium. The Medusa of Velella has, 

 in fact, the essential characters of the Anthomedusae. 



Our knowledge of the life-history of the Siphonophora is very 

 incomplete, but there are indications, from scattered observa- 

 tions, that in some genera, at least, it may be very complicated. 



The fertilised ovum of Velella gives rise to a planula which 

 sinks to the bottom of the sea, and changes into a remarkable 

 larva known as the Conaria larva. This larva was discovered 

 by Woltereck^ at depths of 600-1000 metres in great numbers. 

 It is very delicate and transparent, but the endoderm is red (the 

 colour so characteristic of animals inhabiting deep water), and it 

 may be regarded as essentially a deep-sea larva. The larva 

 rises to the surface and changes into the form known as the 

 Ratarula larva, which has a simple one - chambered pneu- 

 matophore containing a gas, and a rudiment of the sail. 

 In contrast to the Conaria, the Ratarula is blue in colour. 

 With the development of the zooids on the under side of this 



1 Zool. Jalirh. Suppl. 1904, p. 347. 



