228 



THE EYE IN EVOLUTION 



Fig. 234.— The Sea- 

 squirt, ascidia. 



The adult covered by 

 its tunic (test), the lower 

 end attached to a rock, 

 the upper end ending in 

 an inhalant siphon 

 (mouth), and on the mor- 

 phological dorsal surface 

 an exhalant sijshon 

 (atrial opening). Around 

 both apertures thei-e are 

 sometimes jDigment spots 

 of unknown character. 

 During life, the animal 

 draws water in through 

 the first and expels it 

 from the second ; if 

 irritated, water is forci- 

 bly expelled from both, 

 hence the name " sea- 

 squirt." 



The TUNiCATA (urochordata) are typified 

 in the Ascidians or sea-squirts (Fig. 234). 

 Ascidia in its free-swimming larval stage is a 

 tadpole-like creature, about 1"0 mm. in length, 

 possessing the chordate characteristics of a 

 brain and a dorsal tubular nervous system, a 

 notochord and gill-slits. At this stage it is 

 provided with a single cerebral eye associated 

 with a statocyst, but as the hermaphroditic 

 adult settles to its sedentary plant-like life 

 within its thick tunic of cellulose and attaches 

 itself to rocks or weeds, the nervous system is 

 reduced to a single ganglion above the pharynx 

 and the eye disappears. In some of these forms 

 the siphons respond to light by retraction. It 

 is true that pigmented spots are found around 

 the siphonal openings, which used to be con- 

 sidered " ocelli", but in Ciona, at any rate, they 

 are in fact not light-sensitive (Millott, 1957). 



The transient eye of the larval Ascidian is of un- 

 usvialmterest (Kowalevsky, 1871 ; vonKvipffer, 1872 ; 

 Froriep, 1906). It arises as an out-pouching of the 

 cerebral vesicle which forms a single sensory organ 

 consisting of a sac containing a statocyst and an 

 extremely elementary eye on its dorso -posterior wall 

 (Fig. 235). The retina is composed of a few sensory 

 cells derived from the inner wall of the neural tube ; 

 it is capped with pigment and above it lies a rudi- 

 mentary cellular lens. It is interesting that the visual cells are morphologically 

 inverted inasmuch as they face towards the cavity of the sensory vesicle while the 

 intrinsic lens faces towards the brain as if it would be 

 effective only for light traversing the transparent body 

 of the animal. 



In free -swimming Tunicates visual organs may 

 persist ; thvis in the asexual form of Salpa there is a 

 single median horse-shoe-shaped ocellus and some- 

 times smaller accessory ocelli on the dorsal aspect of 

 the animal closely associated with the single nerve 

 ganglion. 



The LANCELETS (ACRANIA ; CEPHALO- 



chordata) are variously regarded as a pioneer 

 off-shoot from the chordate stock or as a 

 degenerate member of the phylum. They 

 possess a dorsal tubular nerve-cord, a notochord 

 and giU slits but lack a differentiated brain or 

 eyes. T" - are typified in the common lancelet. 



Fig. 235. — The Eye of 



THE ASCIDIAN TaDPOLE 



Diagram of the sensory 

 vesicle with a unicellular 

 otolith and an ocellus 

 (above right) with retinal 

 cells, pigment and 3 lens 

 cells situated towards the 

 cavity of the vesicle (Ber- 

 rill, The Origin of Verte- 

 brates, Oxon., 1955). 



