1897.] Cephalic Homologies. 929 
Amphioxus, both by their large number and by their peculiar 
subdivision, by secondary bars, resemble strikingly Tunicate 
gill clefts, but differ strikingly from those in'any of the true 
vertebrates. How far the sexual organs of the cephalochorda 
and urochorda are comparable is not yet clear, but they show 
this striking resemblance that, in both, the sexual elements 
are discharged by dehiscence and pass through the ectoderm 
into the atrium, whereas in all vertebrates the sexual elements 
are discharged into the body cavity, or a duct derived from 
the body cavity. The excretory organs of Amphioxus in their 
association with the gills, and in their exterior or ectodermal 
openings show features which cannot be considered vertebrate, 
but I venture to express the expectation that homologous 
organs will be found in Tunicates. Finally as regards the 
central nervous system, the resemblance, which is almost 
identity, between Tunicates and Amphioxus has been often 
emphasized, but if we follow Howard Ayers in his attempt to 
homologize the parts of the brain in Amphioxus with those in 
true Vertebrates, we shall become only more and more im- 
pressed with the wide divergence between the two. 
There remains the muscular system, which presents a strik- 
ing and genuine resemblance to the metameric musculature of 
fishes, despite the secondarily acquired asymmetry in Amphi- 
oxus. It is a singular coincidence that some of those, who 
consider the segmentation of the body morphologically so in- 
significant, that it has no value to prove the kinships of An- 
nelins and Amphioxus, yet consider it of great importance as 
evidence of the relationship of Amphioxus to the Vertebrates. 
But if we consider Amphioxus as a type intermediate between 
Annelids and Tunicates, the segmented musculature has been 
lost in the latter, an hypothesis which seems to me plausible, 
for as I have pointed out in my essay on “ Senescence and Re- 
juvenation,” all evolution of animals depends not only on the 
acquisition of characteristics, but also very largely on the loss 
of characteristics ; this loss, as exemplified in the gill clefts 
and arches of the higher Vertebrates, affects the early embry- 
onic stages, apparently to allow the embryonic material to un- 
dergo a new development. The assumption that the Tunicates 
