MICROSCOPICAL STUDIES. 51 



Development of Gill Arches. — These are of two kinds ; half 

 are forked at the ventral ends, others which alternate are unsplit. 

 The cleft in front of, and that behind each unsplit rod originally 

 formed together a single opening which, by secondary growth 

 downwards of a portion of the wall, become divided into two in the 

 manner shown in Fig. 16, Plate IV. The down-growing rods may 

 be therefore termed secondary arches, the others primary. In adult 

 individuals connecting bars cross the clefts so as to join more 

 intimately the primary and secondary arches, an arrangement 

 reminding us of that in the Ascidians. 



A comparison of Amphioxus with a typical Ascidian is extremely 

 interesting and suggests such a closeness of relationship that were 

 it not for the emphatic developement of a central skeleton, there 

 would be more reason in classing it as a highly developed free- 

 swimming Ascidian, than as a little developed ancestral fish. 



The points of similarity are well brought out by glancing at the 

 comparative diagrams of the two, shown in figs. 14 and 15. The 

 section shown in both figures is transverse. In the Ascidian, the 

 section is just posterior to the mouth and through the nerve ganglion 

 that serves as brain. That of Amphioxus is through the pharynx. 

 Examination shows how, except in minor details of relative thick- 

 ness and arrangement of the tissues, the fundamental similarity 

 (homology) is absolute, saving for the presence in the lancelet of a 

 central skeleton, or notochord (tit). In both the body is encased 

 in. a more or less structureless cuticle. Within this is a ring of 

 muscular fibres, and more or less sunk in this muscular layer is the 

 central nerve-mass (n.) Suspended from the muscular band at a 

 point just beneath the nerve centre, is a perforated pharyngeal tube 

 shown in figs. 14 and 15, by an interrupted line (ga.) intended to 

 represent the sections of alternate gill arches and gill clefts. Top 

 and bottom of this pharyngeal tube are two grooves, respectively 

 the epibranchial groove and the endostyle. 



The minor differences as seen in such a section are : — In the 

 Ascidian the cuticle is immensely thickened to form a protective 

 test ; but the muscular hoop is infinitely less developed than in the 

 Lancelet, due to the want of much muscular effort in the sedentary 

 life led by the former. In the Lancelet again, the pharyngeal tube 

 is only attached dorsally. In the Ascidian, it is, at the points shown, 

 attached both dorsally and ventrally. 



In both animals the mechanism of respiration and feeding is 

 alike ; the muscular arrangements are strictly comparable ; the 

 reproductive products are in both expelled from the genital glands 

 into the atrium and leave the parent by either the atrial pore or 



