552 THE EYE IN EVOLUTION 



The occurrence of a neuro -endocrine system consisting of well-defined 

 groups of neuro -secretory cells among Invertebrates is widespread. In its 

 most primitive form it is seen in polyclad worms (Turner, 1946), but it 

 becomes conspicuous in the more highly developed Annelids among which 

 neuro -secretory centres are prominent, particularly in the cerebral ganglia 

 where they inhibit maturation of the gametes (Bliss, 1951 ; Durchon, 

 1951 ; Bobin and Durchon, 1952). Neuro -secretory cells have also been 

 described among Molluscs in the central nervous system of Opisthobranchs, 

 Prosobranchs, Scaphopods and Cephalopods (Young, 1936 ; Gabe, 1949- 

 53) ; the part played by simple hormones, probably of the nature of tyramme 

 and betaine, in the regulation of the integumentary hormones of Cephalopods 

 has already been discussed. ^ A similar neuro -secretory function is more 

 common and effective in the nervous system of Arthropods ; indeed, in 

 this phylum which does not possess a closed vascular system, no means is 

 available for free circulation other than the rich hsemolymph supply which 

 bathes the nervous system. In Crustaceans the neuro -endocrine system and 

 its functions have received much attention ; it consists of an x-organ and 

 other groups of cells in the optic lobes and the cerebral and (probably) the 

 first thoracic ganglia, while the storage-release organ is the sinus gland. In 

 Insects the homologous system is the intercerebralis-cardiacum-allatum 

 system. Among Myriapods, in the centipede there is an organ homologous 

 to the x-ORGAN of Crustaceans (de Lerma, 1951) ; while among Arachnids, 

 the chromatophorotropic principle of the nervous system can be correlated 

 with similar neuro -secretory cells (Brown and Cunningham, 1941 ; B. 

 Scharrer, 1941). In Xiphosurans the neuro -secretory system is large and 

 is of peculiar ophthalmological interest in so far as the lateral rudimentary 

 eye of the king-crab, Limulus, as well as the central nervous system, contains 

 neuro-secretory cells (Scharrer, 1941 ; Waterman and Enami, 1954). Among 

 the Proto-chordates, the neural gland of Ascidians secretes agents affecting 

 pressor, melanophore and gonadotropic activities and is thus homologous 

 with the pituitary gland of Vertebrates (Carlisle, 1951). And in the latter 

 phylum the neuro-secretory system reaches its zenith in the hypothalamo- 

 hjrpophyseal complex wherein the posterior lobe of the pituitary is linked 

 with neighbourmg hypothalamic nuclei. In Crustaceans, Insects and Verte- 

 brates the neuro -endocrine system is of sufficient interest to merit special 

 mention. 



THE NEITRO-BNDOCRrNE SYSTEM OF CRUSTACEANS 



Since the discovery cf the small accumulation of neuro-secretory cells 

 lying on the surface of the optic lobe in Crustaceans by Hanstrom (1931) 

 and called by him the x-organ, several other ganglia have been described 

 in the eye-stalks, the cerebral ganglion and possibly in the thoracic ganglionic 

 mass whid 'nave comparable histological appearances and functions ; maps 



1 p. 93. 



