39 o ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



In the latter case the male parent remains on guard in the burrow. In 

 Lepidosiren, where the gills are inefficient and degenerate and pulmonary 

 respiration essential to life, a unique arrangement is found whereby the 

 male is freed from the necessity of deserting its charge and going to the 

 surface to take in a breath of air. This consists in the hind limb becoming 

 converted into a temporary gill with long respiratory filaments richly 

 supplied with blood. These filaments sprout out at the commencement 

 of the breeding season and then rapidly atrophy when no longer required. 

 In rare cases (Fig. 170) the pectoral limb undergoes a similar modifica- 

 tion. The egg develops into a larva of a type closely resembling that 

 of the next group the Amphibia. 



The larva (Fig. 171), except in Ceratodus, is provided with four 



rnal gills on each side, projecting from visceral arches III, IV, V 



and VI. Each gill is when fully developed pinnate in form, is richly 



supplied with blood the aortic arch being diverted out into it in the 



FIG. 170. 

 Male Lepidosiren showing respiratory limbs. 



form of a loop, and it is provided with muscles by which it can be 

 flicked actively so as to renew the water in contact with its surface. 

 In all these three features the external gills agree with those of Cross- 

 opterygians and Amphibians. The external gills are the main organs of 

 breathing during larval life but they eventually, sooner (Lepidosiren) or 

 later (Protopterus), disappear completely. 



One of the most interesting problems connected with the evolutionary 

 history of the Vertebrata is that of the origin of the two pairs of limbs 

 which constitute such a characteristic feature of the group. How did 

 the limbs make their first appearance ; from what pre-existing organs 

 have they evolved ? Whatever was the function of the forerunners of 

 tin- limbs we are probably justified in believing (i) that they were organs 

 which projected beyond the general surface of the body and (2) that they 

 freely movable by means of muscles. Now the external gills are 

 the only organs of archaic vertebrates which fulfil these conditions 

 lactorily. There is some reason to believe that there were present 

 in the primitive vertebrate a series of external gills extending back 

 behind the region of the body which in the modern vertebrate is con- 

 cerned with the function of respiration. And it has been suggested that 



