1218 



SCAXiJIXAVIAN KI.SIIKS. 



basket. Some of tlie fii'st-foniR-d gill-openings close 

 again; and the jmsterioi- |)art of the tiranchial l)asket 

 has its slits synmietrieally arranged after the termina- 

 tion of the larval period. The month appears on the 

 left side, first a.s a fine slit, afterwards as a large, el- 

 liptical apertnre with tumid margin (fig. 37.5). A true 

 anus opens, also, as a rule, on the left side, and the 

 original gastrula mouth grows together, the connexion 

 between the intestinal canal and the cerebrospinal canal 

 being thus cut oft'. As yet the larva moves only with 



wards and forwards ovei- the tops of the branchial slits. 

 The latter {end, with its supporting rods, the so-called 

 eiuJostijJc, with a right and a left part), on the other 

 hand, follows, according to Willey, an opposite direction 

 of development, backwards from in front, originating in 

 a thickening, at first transversal (figs. 37.5 — 379, end), 

 of the intestinal \\all beside the club-shaped gland. 

 The first rudiment of the atrium appears (fig 380, A, 

 sar) where the atriopore afterwards has its place, and 

 presents the appearance of two lioi'izontal ridges, grow- 



rnd ,/l I II III l\ 11/ \ mix ep 



v'li 

 Kig. 376. A larva .S'/.^ iiiiii. lung, t:iki.-ii swiinmiiig freely in tlie sea, between 1 and 4 fathoms below the surface, at Kristineberg (Bolius- 

 lan), August 17, 1894. Seen from the left. Letters and numerals as in the preceding figure; besides: am, the ciliated mouth of the cerebro- 

 spinal canal; eji, the forward growtli on the inside of the intestine, a fold originating from the inferior wall of the intestinal canal. Number 

 of tlie myomeres here 61. Gill-slits and their rudiments visible liere to a number of l.S (/ — XIII). 



the aid of tlie dermal cilia; but it has begun to liend 

 its body, since the rostral and caudal fins have become 

 sharpened. Over the gill-slits there grows from above 

 (fig. 377) a longitudinal fold fr(jm the walls of the 

 body (soraatopleures, the outer parts of which grow 

 into the so-called metapleures or abdominal fins, met. 



Fig. 377. Forepart of a larva 3".^ """• 'cug, seen from the right. 

 After Kay Lankester and Willey. Here it appears how the wall of 

 the body from the ninth gill-slit forward lies like a leaf outside the 

 upper parts of the slits, and further forward is continued in a sharp, 

 free edge, forming an upward curve on tlie right side of the head. 

 Letters ami numerals as in the preceding figure. 



figs. 379 and 380) on each side, fii'st on the right; 

 and })etween these folds the atrium is formed, first only 

 on the ventral side and growing forwards from l)ehind. 

 The construction of the branchial basket is contiiuied 

 (figs. 376, 378, and 379) by the addition of epibran- 

 chial and hypobranchial grooves. The former {ep) ori- 

 ginates, so far as we have been able to see, as a fold- 

 like inciu'vature, advancing in its development forward 

 from behind along the lower wall of the intestinal canal, 

 which at the hindmost rudimentary gill-.slit grows up- 



ing to meet each other (fig. 380, B), on the inside of 

 the above-mentioned dermal folds {met), whose inferior 

 margin subsequently becomes on each side of the body 

 a longitudinal lateral ridge (abdominal fin or meta- 

 pleure). Afterwards the atrium expands upwards, en- 

 croaching upon the region of the abdominal cavity on 



fr VII- I- ril fr 



I J .,,1"'' 



1 Mil III 



Fig. 378. Anterior end of a somewhat larger larva, seen from the 



right. After Willey. Here a series of seven secondary gill-slits 



(/- — VIP) has appeared on the right side, above the inferior ends 



of the ]irimary slits (/ — XIII), which have been translated for the 



most part to the left side, and among which the thirteenth (hindmost) 



has been obliterated, and the first has almost suffered the same fate. 



The endostyle (end) has grown backwards, and its posterior extremity 



extends behind the club-shaped gland {gl). Further forward the velum 



of the mouth cavity (vel) has begun to develop. In the skeletogenous 



layer (cliordal sheath) a series of so-called fin-rays {fr) has appeared. 



On the atrial floor tw'o so-called renal organs (r) may be seen. Letters 



and numerals otherwise as in the preceding figure. 



the sides of the branchial l)asket, and continued within 

 the abdominal cavity even behind the atriopore, beside 

 the intestine. 



The details of this development should be easy of 

 comprehension with the aid of the figures which we 



