58 ALLMAN. — USE OF THE TOWINO NET. 



The Crustacea are also rich in larval forms which abound in 

 the most superficial zone of the sea, where their development is 

 favoured by the more intensified conditions of light and aeration 

 to which they are there exposed. Among those larval forms are 

 the free-swimming young of the higher Crustacea, especially those 

 known to the older observers under the name of Zoea, at a time 

 when these immature organisms were regarded as completely 

 developed and independent forms. Also the larvas of the Bar- 

 nacles, a low section of the Crustacea, which, though absolutely 

 fixed in their adult state, spend the early period of their lives as 

 free-swimmers in the open sea. These are all active creatures 

 of singular, and often grotesque aspect, and are among the most 

 frequent captives of the towing net. Many of them are of great 

 interest in their bearing on the laws of development and on the 

 affinities of groups. They admit of being well preserved in spirit. 



Mollusca, 



Among the most abundant and striking pelagic forms are the 

 SalpaB belonging to the low moUuscoid group of the Tunicata. 

 They are of a somewhat oval or prismatic shape, attaining a length 

 of from half an inch to two or even three inches ; they are of crystal- 

 line transparency, with usually a large brown, reddish, or purple 

 globular body visible within them near one end, and caused by the 

 location at this spot of some of their more important viscera. They 

 swim in jerks near the surface of the sea, either singly or united into 

 long chain-like groups. Being of considerable consistence notwith- 

 standing their clear gelatinous appearance, they can generally be 

 very well preserved in spirit ; but great care must be taken not 

 to separate the components of the chain-like series which are very 

 easily detached from one another. Indeed when taken in the 

 towing net these are very often found to be broken up, and the 

 safest way of capturing them is by gliding under them as they 

 swim past the vessel a perforated ladle, as has been recommended 

 in the case of the Siphonophora, and then carefully transferring 

 the whole chain to spirit. 



Appendicular ia is another tunicate also frequently taken in the 

 towing net. It is a minute clear oviform creature, of about the 

 size of a millet seed, and easily recognized by a rapidly vibrating 

 transparent ribbon-shaped swimming organ, somewhat resembling 

 the tail of a tadpole, and -springing from a point near one end of 

 the body. It may be transferred to spirit by means of the dip tube. 



Holding a much higher position among the Mollusca are the 

 Pteropoda. These are free-swimming animals provided with a 

 pair of wing-like appendages by the aid of which they flit through 

 the superficial zone of the sea. They are usually clear-bodied, 

 and either colourless or tinged with some shade of purple, and 

 generally attain a length of from half to three quarters of an inch. 

 Some of them are said to be diurnal in their habits, sinking into 

 the deeper regions during the night, while others are believed to 

 be nocturnal and to withdraw themselves from observation during 

 the day. These statements, however, require confirmation. Some 



