iv PHYLUM CCELENTERATA 221 



generations ; but in some cases development is accompanied by a 

 well-marked metamorphosis. 



The Ctenophora are divisible into four orders as follows: — 



Order 1. — Cydippida. 



Ctenophora having two tentacles, retractile into sheaths, and 

 unbranched meridional and stomodreal vessels. The body is 

 either circular in section or is slightly compressed in the trans- 

 verse plane (Figs. 158 and 168). 



Order 2. — Lobata. 



Ctenophora having numerous non-retractile lateral tentacles 

 contained in a groove : the bases of the two principal tentacles are 

 also present, but have no sheaths. The stomodoeal and meri- 

 dional vessels unite with one another. The body is compressed in 

 the transverse plane, and is produced into two large oral lobes or 

 lappets and into four pointed processes or auricles (Fig. 169). 



Order 3— Cestida. 



Ctenophora having a band-like form, owing to the extreme 

 compression of the body in the vertical plane. The bases of the 

 two principal tentacles are present, enclosed in sheaths, and there 

 are also numerous lateral tentacles contained in a groove. Union 

 or anastomosis of the meridional and stomodaeal vessels takes 

 place (Fig. 170). 



Order 4.— Beroida. 



Ctenophora having no tentacles. The mouth is very wide, and 

 the gullet occupies the greater part of the interior of the body. 

 The meridional vessels are produced into a complex system of 

 anastomosing branches (Fig. 171). 



Systematic Position of the Example. 



Hormiphora plumosa is a species of the genus Hormiphora, be- 

 longing to the family Pleurobrachiidm and to the order Cydippida. 



The presence of two tentacles, retractile into sheaths, and of 

 unbranched meridional canals places it in the order Cydippida. 

 In this order there are three families, amongst which the Plcuro- 

 brachiidoz are distinguished by the absence of any compression of 

 the body, the transverse section being circular. The genus 

 Hormiphora is distinguished by having a rounded body somewhat 

 produced at the oral pole, and by the aperture of the tentacle- 



