598 



DEPTHS OF THE OCEAN 



life fix themselves to the bottom and become sessile, like the Hydro- 

 medusae, forming colonies by budding. They are 

 thus meropelagic, whereas all other Tunicata are 

 holopelagic and perfectly independent of the 

 bottom. These latter are the only ones to be dealt 

 with here, viz. Appendicularians, Salp^e, and the 

 genera Dolioluui and Pyrosonia. 



The Appendicularia resemble greatly the larv« 

 of Ascidians, and present a remarkable likeness to 

 early vertebrate types. As a rule they are trans- 

 parent and perfectly devoid of colour. Their body 

 (see Fig. 445) is clumsy in shape and contains all 

 the organs of nutrition and propagation, with a 

 long elastic tail which serves solely the purpose of 

 locomotion. Lohmann has studied the biology of 

 this group,' and his results will be referred to later. 

 The Appendicularians live mostly in the upper 200 

 metres of the ocean, though in tropical waters they 

 occur deeper ; in fact in the Sargasso Sea the 

 German Plankton Expedition found more of them 

 below than above 200 metres. As with most sur- 

 face forms the species are most abundant in warm 

 waters, like Appendicularia sic2ila,Fritillaria ve^iusta, 

 and Oikopleura parva, while Oikopleura vanJioffeni 

 and O. labradoriensis are northern forms. 



The Salpae are free-swimming, barrel-shaped, 

 transparent animals, well-known to all sea-faring 

 people (Fig. 446), They are often seen crowding 

 the surface-waters of the ocean in countless num- 

 bers. Among investigations of recent years we 

 may cite the report on the " Valdivia " collection 

 by Apstein.^ In hauls with closing nets the 



" Valdivia" found the majority of Salpae in depths less than 200 metres. 



Fic. 445. 



Oikopleura labradoriensis 



Lohm (about Y")- 



(From Lohmann.) 



Fk;. 446. 

 Salpajusiformis forma aspera, Cham. Nat. size. 



Lohmann, Ergeb. Plati kton- Expedition , Bd. 2, 1896. 



Apstein, IViss. Ergeb. '^Valdivia" Expedition, Bd. i; 



[906. 



