I'LANKTONIC LARVAE 



Not all marine molluscs have a planktonic stage and indeed some 

 quite closely related types differ, e.g. winkles have but whelks do not have 

 them. The earliest planktonic stage of the univalves (Gasteropoda) is a 

 'trochophore' very like that of a polychaete worm but instead of segmenting 

 it gives itself a peculiar twist in what is called the 'veliger' stage (Fig. 24 ; 

 4-7: Plate XXI) and it then develops its shell which is usually spiral. In 

 some the trochophore stage lives inside the egg capsule and only the veliger 

 is planktonic, and in some (e.g. the whelks) both trochophore and veliger 

 live inside the egg and they emerge as tiny, shelled adults. 



The bivalves (Lamellibranchiata or Pelecypoda) also have a trochophore 

 and a veliger stage but after metamorphosis the little bivalves with shells 

 already formed (Fig. 24; 8-12) sink only slowly to the bottom, often in 

 immense numbers, and are thus frequently taken in the plankton. 



Most of the more primitive types of marine Crustacea are planktonic all 

 their lives and so have been considered in the previous chapter, but this 

 chapter must refer to the larvae of the attached barnacles and the bottom hving 

 shrimps, prawns, crabs and lobsters. 



The attached acorn barnacles are not obviously related to the crabs, but 

 any doubts about their relationships are at once settled by the planktonic 

 larva, which is a typical nauplius (Fig. 21 ; 5: Plate XXIV) very like those of 

 the copepods dealt with in Chapter 5 (Fig. 21 ; 1-4). This swims freely in the 

 plankton, moulting and adding more and more appendages just as do the 

 copepod nauplii, but after the fnial moult it develops simple bivalve shells 

 reminiscent of the ostracods (p. 65) and it is then called a 'cypris' stage (Fig. 

 21 ; 6: Plate XXIV). This gradually sinks out of the plankton and if it lands in 

 a suitable place it becomes attached and metamorphoses into the typical 

 barnacle — though 'landing in a suitable place' could itself form a most 

 interesting chapter in a book. 



Euphausids (p. 74) also have a naupUus larva (Fig. 21 ; 10) but this gradu- 

 ally changes, via a long series of moults (Fig. 21 ; 11) which are named accord- 

 ing to the development of the appendages, into the adult. All, including of 

 course the adult, are planktonic. 



The normal practice in the bottom-living decapods is for the female to 

 carry her eggs, usually underneath the abdomen (e.g. the berried lobster) 

 or under the carapace between the paired legs. The first stage of development 

 — a nauplius — is often passed inside the egg shell although there is a free 



9. Myriochele danielsseni, young larva. 



10. Nepthys coeca, nectochaete stage. 



11. Magelona papilUcornis , old larva. 



12. Pcctinaria aiiricoma, (a) young larva; (/)) old larva in a gelatinous tube. 



13. Lattice conchilega, old larva in a gelatinous tube. 



[All from Thorson.] 



87 



