ECHINODERMATA OF NEW ENGLAND. 13 



near the rim. They sometimes pass directly from centre 

 to rim, at other times subdivide, bifurcate and coalesce. 

 Different tubes in the same bell may have a straight or a 

 tortuous, or a marginal course. The movement of a nutri- 

 tive fluid in the tubes can be seen through the bell-walls. 

 Bell-cavity present or wanting. When present, it is some- 

 times partly closed at its entrance by a washer-like body 

 called the velum. 



The bell margin of the Hydrozoa is either entirely cre- 

 nated, slightly notched or scalloped. Small, transparent 

 cells, the sense capsules, otocysts, with enclosed calcareous 

 grains called otoliths, are commonly present. The number 

 and structure of these organs vary in different genera. In 

 the Hydrozoa, when present they are placed around the 

 bell margin and their number is from four to sixteen ; in 

 the Ctenophora there is, in the adult, a single polar sense 

 capsule. The sense-capsules of the former group are par- 

 tially covered on the upper side by a small, gelatinous lap- 

 pet which is called the " hood." Jelly fishes which have a 

 hood are called the " hooded-eyed" ; those without, the 

 " naked-eyed" Medusae. 



Small, thread-like bodies, called tentacles, varying in 

 number and size, hang down sijgly or in clusters from the 

 under side of the body at or near the bell-margin. In 

 those genera (Ctenophora) which have a single polar sense 

 capsule opposite the mouth, there are either two long ten- 

 tacles with side branches with numerous smaller body fil- 

 aments, or these structures may be wholly wanting. 



These animals are generally small, transparent, phos- 

 phorescent in darkness when the water is agitated. Many 

 are highly colored. Water forms the great mass of the 

 body substance. Their larval forms are among the most 

 abundant animals found on the surface of the ocean. The 

 Hydrozoa are classified as follows : 



Order I. Hydroida. Bell-shaped bodies, without flap 



