ZOOPLANKTON II 



has a very tough jcUy which is not easily disintegrated and thus in spite of 

 its warm habitat it too can reach Norway. Its tough jelly is taken advantage 

 of by an amphipod called Pliroiiiiiia wliich eats its way into the salp, leaving 

 nothing but an empty barrel of jelly in which it continues to live as a pro- 

 tective house (Plate XIX). Salps feed only on phytoplankton which they 

 filter off from a stream of water they pass through their insides as they swim 

 by the rhythmic contraction of their muscle bands. They have an interesting 

 life history with an 'alternation of generations'. Single individuals, the 

 solitary generation, produce a stolon or ribbon of buds in a long 'chain' 

 which may give rise to several thousands of chain forms, the aggregate 

 generation. The growth and release of the chains in Thalia take only ten 

 days. The aggregate salps stay together in this loosely attached chain 

 until broken off by waves or other disturbances in the water. Each individual 

 is a sexual form and after fertilization one or more 'buds' inside the body 

 grow to form little embryos of the solitary generation, to be released on 

 the death of the mother (Plate XXI). Thus a few of the solitary salps 

 transported by the currents can give rise to great numbers of aggregate 

 salps if the conditions are suitable. The solitary and aggregate individuals 

 look so different that at first they were given separate names but it was 

 soon realized that they were different forms of the same species, and the 

 two names were then hyphenated. This clumsy terminology was later 

 dropped but some of the olcier books still read today use names like Salpa 

 niiiciiiata-fiisiforiiiis and Salpa iiiticroiiata-deiiiocratica. This latter species, now 

 called Thalia {Iciiioctatica (Fig. 23 ; 3 and 4) is probably the most abundant 

 salp in the world, but only now and then are conditions suitable for it to 

 reach the British Isles. It occurred in numbers in Plymouth Sound in 1893 

 and off the Hebrides in 1886, 1904 and 1958 but has not yet been found 

 farther north. Occasionally a specimen of the larger tropical form is found 



4. Tlialici democratica; aggregate stage with a single embryo {b) which will eventually become a 

 free solitary stage. 



5. Salpa ftisijoniiis; aggregate stage. 



6. Salpa fiisiformis; solitary stage, with stolon. 



7. Dolioletta gegeubanri; gonozoid, complete with sexual organs. The eggs trom this stage hatch 

 into 'tadpoles' (cf. Plate XX). 



8. A later stage of the 'tadpole' showing the remains of the outer capsule and the oozoid 

 developing inside. 



9. The late oozoid or 'old nurse' now devoid of almost all its internal organs, but with broad 

 muscle bands and balancing organ (statocyst), a nerve centre and the remains of the dorsal 

 process. 



ID. A view from above of the oozoid at its functional stage with a prominent dorsal process. 

 (a) the buds developing on the stolon (/;) the buds migrating to the dorsal process, (c) double 

 rows of lateral zooids, the trophozoids which serve only to catch food for the whole, (d) 

 median rows of phorozoids, and (e) the youngest migrating buds which will become new 

 gonozoids. 



II. A later stage of a phorozoid, now broken free from the dorsal process of the oozoid and acting 

 as a bearer for two developing gonozoids. These will eventually become the free hving sexual 

 forms figured in 7. 



79 



