LATERAL LINE ORGANS. 121 



an auditory organ, possibly homologous with the pectines of scorpions, is con- 

 trary to well established facts. 



Lateral Line Organs of Vertebrates. Summary and Comparison. 

 The lateral line organs of vertebrates consist of several distinct groups that arise 

 at an early embryonic period from the neural surface of the head. Each line of 

 organs makes its appearance as an oval thickening of the ectoderm, located be- 

 tween the dorsal extremity of a gill arch and the lateral margin of the medullary 

 plate. (Figs. 26-34.) The thickening gradually extends in a peripheral direc- 

 tion, and as it does so it separates into a superficial linear series of sense buds 

 and an accompanying underlying nerve and ganglion. Subsequently an in- 

 folding of the ectoderm may take place along the line of growth, forming first an 

 open groove and then a canal, in which the organs are located at regular intervals. 

 Finally the several canals may unite, forming a continuous system, but each part 

 that was originally a distinct canal is innervated by a special cranial nerve. 

 (Fig. 89.) 



It has been suggested that the anlage of each canal represents a very ancient 

 sense organ (the so-called branchial sense organ), but so far as I know, no ex- 

 planation has been offered for the extraordinary fact that these ancient organs 

 must have originated, not around or close to the vertebrate mouth, as one would 

 naturally suppose, but from the opposite or aboral side of the head; and not from 

 a single anlage, but from several. 



This condition, however, is perfectly intelligible as soon as we recognize that 

 the whole system of taste buds and lateral line organs of vertebrates represents 

 the thoracic and vagal coxal sense organs of arachnids, which there lie on the neural 

 surface of the head around the primitive mouth, the latter having closed up and 

 disappeared in the vertebrates. 



When the sense organs of the arachnids are projected on the neural surface 

 of the cephalothorax, the principal groups, each one containing many organs, 

 appear as oval or circular areas arranged around the mouth. (Fig. 89, A.) We 

 may recognize three sets : the gustatory organs and slime buds located side by side 

 in the thoracic and vagal appendages, and the chemotactic general cutaneous 

 organs located on the neural flanks of the thoracic and branchial regions, and 

 supplied by a great longitudinal nerve arising from one of the anterior thoracic 

 neuromeres, l.n. 



Taste buds predominate in the second, third, fourth, and fifth thoracic coxae, 

 i.e., those immediately surrounding the mouth. Sense cells of the same general 

 type are abundant in the flabellum and in the vagal appendages, but there they 

 may serve as tactile organs, or for some other purpose. 



The organs of the gustatory-tactile type and the slime buds may arise side 

 by side from the same anlagen, and they may be supplied by the same nerve 

 trunks and ganglia. Their later phylogenetic history appears to follow along the 

 same lines in both cases, but there is apparently a tendency to separate, more and 



