Eyes of Molluscs and Arthropods. 665 



dermic cells. the iucrease iu number of nervous cells arising from that 

 point goes band in band witb tbc increasing- scusiti vcness of tbe organ, 

 and tiually givcs rise to a subjacent biyer of ganglionic cells united ou 

 tbe one band witb tbe central nervous system, aud on tbe otber witb tbe 

 sensitive cells, between wbicb tbe ganglionic ones bave arisen. 

 Tbe only difference between tbe so-called sense cells and tbe ganglionic 

 ones is tbat tbe former retain to a greater degree tbeir power of receiv- 

 ing Stimuli directly from tbe stimulating agents. wbile tbe latter, 

 witb increased sensibility, receive only tbose Stimuli wbicb bave 

 acted upon tbe former, and conduct tbem to tbe central nervous system. 

 We sbould, tberefore, expect to find, and do find even in bigbly devel- 

 oped sense organs, many transitioual stages between sensory, and 

 ganglionic cells. 



In tbe ontogenetic development of tbe bigber sense organs, we uo 

 more find a repetition of tbe pbylogenetic process, tban is found in tbe 

 central nervous system. Tbere is an invagination , or tbickening, 

 of tbe place in question, witb a Separation «eu masse« of tbe ganglionic 

 and sense cells, and a subsequent outgrowtb from tbe nervous centre to 

 join tbe sense ganglioü. Tbese two processes , tbe Separation of tbe 

 ganglionic cells and tbeir union witb tbe central nervous system . are 

 easily studied in tbose organs wbicb are widely separated from tbe 

 latter. But wben tbe two structures originate from tbe same 

 place, as iu tbe eyes and brains of Arthropods, tbe origin of tbe brain 

 and tbe optic ganglion, and tbeir union witb eacb otber, may, ontogene- 

 tically, form a single process. Tbe united ganglionic cells, wbicb bave 

 arisen witb tbe sense cells, form tbe optic ganglion (Cepbalopods, 

 Worms, and Arthropods'. Tbe optic ganglion may be united witb 

 tbe eye by a primary, ganglionic optic nerve, and witb the 

 brain by a secondar y optic nerve, or outgrowtb of tbe central 

 nervous system. But, if tbe eye and brain are closely united, then tbe 

 optic ganglion will fuse witb tbe latter. and tbe secondary optic nerve 

 disappear. 



Chapter IV. Oeneral Remarks upon the Arthropods. 



Our studies bave sbown that the eyes of Arthropods, like tbose of 

 Molluscs . consist of ommatidia , tbe structure of wbicb is dependent 

 upon tbe shape of tbe eye. 



Tbe compound Arthropod eye consists of a double layer of cells ; 

 the ommateum, and tbe corneal hypodermis; tbe latter has been 



