154 THE PINEAL ORGAN 



invertebrate animals, one cannot fail to be impressed with the essential uni- 

 formity of type, in the general appearance and structure of these larvae, not- 

 withstanding the wide differences which exist in the adult animals. In 

 illustration of this we need only refer to the trochophore larvae of the 

 Brachiopods belonging to the phylum Molluscoidea, those of certain of the 

 Annulata and of the Rotifers (see pp. 88, 91, 92, Figs. 56, Chap. 7 ; 58, 59, 

 Chap. 8). In contrast with the similarity in form of the larvae of these very 

 different types of animal, it is especially noticeable how changes in the 

 habits and conditions of the adult animal are associated with adaptive 

 changes in special organs, e.g. the development of the highly complicated 

 faceted eyes of some insects, which replace the simple eyes of the larva, 

 e.g. in Dytiscus marginalis (Figs. 78, 79 and 80, Chap. 11, pp. 117, 118). 

 The simple eyes of the larva which were present before metamorphosis 

 had taken place while the larva was living in the water, being followed by 

 an aggregate or composite type of eye, adapted to the special needs of the 

 fully developed animal, which is capable of living on land or flying quickly 

 in the air. These changes occur specially in the higher and more recently 

 evolved types of animal, as compared with the more simple and ancient 

 types of shell-fish which we have been considering previously. More- 

 over, as we have pointed out in a previous chapter (p. 26), along with the 

 specialization of the lateral eyes of these more recent types there has in 

 some cases been a regression of the paired median eyes, accompanied by 

 fusion of these in the median plane and presumably also diminution or 

 loss of function. Moreover, as we shall see later, palaeontological evidence 

 indicates that similar regressive changes have taken place in the paired 

 median eyes of vertebrates. 



The phylum Mollusca includes bivalved shellfish such as the mussels, 

 cockles, and oysters belonging to the class Lamellibranchiata (Pelycypoda) ; 

 the Chitons or coat-of-mail shellfish (Amphineura) ; animals with 

 spiral univalve shells, including whelks, snails, and slugs (Gastropoda) ; 

 the elephant's-tusk shells (Scaphopoda) ; and the cuttlefishes, squids, 

 octopods, and Nautili (Cephalopoda). 



The eyes, as we have already mentioned, vary markedly both in position 

 and in structure. Thus, they may appear in the usual place on either side 

 of the fore part of the head ; on the back, as in Chiton ; or around the 

 edge of the mantle, as in the scallop or in the thorny-oyster. They may 

 be of the simple, upright type or inverted. The lens may be cuticular or 

 cellular in type. The simple eyes may be open pits without a lens or the 

 pit may be constricted off from the surface and form a vesicle covered by a 

 " secondary cornea " and enclosing a vitreous or a cellular lens. The 

 eyes may be absent in the adult animal, but a pair of ocelli may be present 

 in the trochophore larva or in the veliger stage of development. In the 



