650 COMPARATIVE ANATOMY 



a simple upright retina. The resemblance holds, it is asserted, for the 

 finer histological details of the retina. 



Gaskell is much impressed by the evidence of the similarity of the 

 skeletal elements of limulus and ammocoetes. "The evidence of the 

 origin of the cartilaginous skeleton of the vertebrate points directly to 

 the origin of the vertebrate from the Palaeostraca, and is of so strong a 

 character that, taken alone, it may almost be considered as proof of such 

 origin." Branchial cartilaginous bars are found in both. The branchial 

 skeleton of ammocoetes begins where that of the arachnids, represented 

 by limulus, leaves off. 



The branchiae of ammocoetes are homologized with the branchial 

 appendages of limulus, which are "sunk-in" during ontogenesis. Since, 

 as a matter of fact, the branchial appendages of limulus are covered with 

 ectoderm, while the epithelial covering of the gills of cyclostomes is 

 endodermal, morphologists find it difficult to accept this homology. 

 Germ layers, however, mean little to Gaskell. 



The cranial nerves of vertebrates are identified with those which 

 innervate the branchial appendages. 



Gaskell has more difficulty in finding in vertebrates a homologue of the 

 limulus heart. Since the heart of limulus is dorsal, a new heart has to 

 be formed when arachnids become vertebrates. Gaskell assumes, there- 

 fore, that a pair of ventral venous sinuses in limulus unite to form a median 

 ventral heart in vertebrates. This change is recapitulated in the onto- 

 genesis of the vertebrate heart. The remnants of the limulus heart are 

 found by Gaskell in the so-called fat-column, which lies dorsal to the 

 spinal cord in ammocoetes but is lacking in higher vertebrates. 



Gaskell appreciates the difhculty involved in attempting to derive 

 the heart and circulatory system of a vertebrate from that of an arthropod. 

 He calls attention to the fact that the invertebrate heart is systemic and 

 drives the arterial blood directly to the different organs of the body. 

 But the vertebrate heart is branchial and drives the blood to a ventral 

 aorta from which it is carried to the gills. The distributing systemic 

 vessel of vertebrates is the dorsal aorta, not the heart, which belongs 

 essentially to the ventral venous system. The invertebrate heart is, 

 therefore, not homologous with the vertebrate heart and the vertebrate 

 heart is new. 



Gaskell derives the vertebrate thyroid gland from the uterus of the 

 paleostracan ancestor. This homology is made without regard to the 

 fact that the thyroid is endodermal in origin while the crustacean uterus 

 is ectodermal. The endocrinal influence of the thyroid upon the gonads 

 is cited as evidence in favor of this homology. 



The hypophysis of petromyzon, with its associated olfactory pits, 

 is compared by Gaskell with the olfactory tube of scorpions. Coelomic 



