Chap. I. BRACHIOLAR ARMS. 23 



anal extremity is the lowest, and the oral extremity inclined to the vertical ; in this 

 attitude they often remain a long time, drifting about with the currents; their only 

 movements being the expansion and contraction of the oesophagus, and the play of 

 the arms. The movements of the arms are exceedingly graceful ; comparatively 

 longer and more slender than the tentacles of the Tubularians, they have none of 

 the stiffness of their movements, the constant curving and thrusting in every direc- 

 tion reminding us rather of the motions of the tentacles of Phyllodoce and similar 

 Annelids. They are never at rest, being always kept in motion to produce currents 

 round the mouth of the larvse ; and, in addition to the action of the powerful 

 vibratile cilia placed round the mouth, are continually bringing fresh water into the 

 oesophagus. 



The large triangular mouth (PI. "iV Figs. 1, 2, 4, m ; PI. VII Fig. 8) opens into a 

 rectangular pouch (PL IV. Fig. 4, m', m"), extending back from its posterior edge ; 

 from this pouch the oesophagus tapers rapidly, and attains, near the apex of the mouth, 

 the size (o) which it retains till it joins the stomach. The surface of the oesophagus 

 (o) presents a more or less corrugated appearance near its junction with the digestive 

 cavity, owing to the somewhat greater thickness of the walls (PI. IV. Fig. 1). 



Brachiolar Arms. — The brachiolar arms (//,/") are appendages belonging only to 

 adult larvas. Our larva has three of them (PL IV Figs. 1, 4, 5, 6), one pair (//), and 

 a somewhat larger odd arm (/"), placed at the base of the odd anterior arm (e 6 ) ; the 

 branches of the water-system terminating in these arms, proceed from a large pouch 

 (»') in the oral extremity (PL IV Fig. 4). The brachiolar arms are, like the others, 

 tipped with orange, but have, in addition, wart-like terminal appendages, each having 

 six to eight nipples, according to the age of the larva (PL IV. Figs. 4, 5, 6, 8 ; PL VII. 

 Fig. 8). These knobs give to the short arms the appearance of the hind feet of 

 Sphinx larvse. In the hollow between the base of the brachiolar arms there is a 

 small elliptical disk (/'", PL IV. Figs. 4, 5, 6 ; PL VII. Fig. 8), reminding us of the 

 madreporic body of a Starfish, and a row of similar disks, two or three on each side of 



the body of the embryo is not loaded down at one Miiller, to facilitate the comparison with his figures, 

 extremity by the young Starfish, thus compelling the and have given one figure of an adult Brachiolaria, 

 larva to assume always one and the same general in its natural attitude (PI. VII. Fig. 8), with which 

 attitude when in motion. It is more common, in the others can be readily compared in their theoret- 

 the younger stages, to see the embryo moving with ical position. The figures here given are drawn 

 the anal extremity uppermost ; it would be as un- from the larva? as they appear swimming through 

 natural to turn these younger stages upside down, as the water ; and I have endeavored, as much as pos- 

 it would be to represent an adult larva in anything sible, in representing them, to give an accurate idea 

 but its .natural attitude (PI. VII. Fig. 8) with the of the mobility of the arms; avoiding, in this way, 

 anal extremity downward. I have therefore com- the unnatural stiffness which characterizes drawings 

 promised, by representing all the stages in the same made under compression, like the majority of those 

 position in which they are generally represented by of Miiller. 



