TUNICATA. 



1223 



At each side of the base of the group of ten- 

 tacles, a little prominent lobular process is 

 present. Lastly, the side of the body, opposite 

 to that on which the tail is placed, becomes 

 somewhat strongly embossed near its pos- 

 terior extremity, and towards the space where 

 the above-mentioned black points occur {Jig. 

 784. B). 



The ovum ready for exclusion differs ap- 

 parently little from the foregoing state. The 

 two anterior styliform appendages have almost 

 entirely disappeared, and the three obtuse 

 processes are further developed. The trunk 

 is contracted towards its anterior extremity, 

 and the yolk-mass still further condensed at 

 the centre of the ovum. 



The external membrane of the ovum becomes 

 excessively thin, and then breaks and allows 

 the embryo to escape. Generally this ex- 

 clusion takes place in the interior of the cloaca, 

 but sometimes not until the ova have passed 

 out by the anal orifice. However that may 

 be, the young animal, free from its envelopes, 

 soon extends its tail, and swims in the ambient 

 fluid by the aid of its undulatory movements. 

 In its general form the young Ascidian re- 

 sembles somewhat that of a newly born tad- 

 pole (fig. 784. c) ; but it still more resembles a 

 Cercaria. The trunk or body of the larval Poly- 

 cl'mum is oval and rather depressed. The 

 whitish tissue of the future integument occupies 

 all the surface, and is considerably developed 

 at the margins ; its substance is granular and 

 apparently subgelatinous ; its consistence is 

 greatest at the surface; and it does not appear 

 to possess a membranous investment. To- 

 wards the centre of the trunk is a large ellip- 

 tical membranous sac, the internal tunic of 

 Milne-Edwards; this is filled with the yellow 

 substance of the vitellus, and is continuous 

 anteriorly with the three tubes dilated at the 

 end, and terminating at the anterior wall of 

 the egg in a sort of sucker. By means of 

 gentle graduated pressure, some of the yolk 

 may be easily made to pass from the principal 

 sac into these appendages, and vice versa ; the 

 little capsule, also, terminating each of these 

 appendages can be made to protrude exter- 

 nally by the same means. At the base of these 

 three processes the vestiges of the others 

 formerly occupying the intervening spaces may 

 be observed. The yellow substance contained 

 in the internal tunic appears to be separated 

 into two portions; the one is clear, and situated 

 near the wall of this sac, and the other, denser 

 and of a deeper tint, occupies the centre. Pos- 

 teriorly a little marginal space, clearer than the 

 neighbouring parts, is also visible, and on one 

 of the sides the above-mentioned minute black 

 spots are visible. The tail is very large, and, 

 like the trunk, is composed of two distinct 

 parts ; the one superficial, colourless, diapha- 

 nous, gelatinous, and much resembling the al- 

 bumen of the eggs of frogs ; the other, central 

 and yellow. This latter part is continuous 

 anteriorly with the central sac of the trunk, 

 and is also composed of a membranous tunic, 

 enclosing a yellow granular and semi-fluid 

 substance. It sometimes appeared to have a 



central canal. The larva?, after swimming about 

 with an active wriggling motion for a few hours, 

 attach themselves to the surface of a solid 

 body, and, if disturbed from their position, 

 swim about as before until they meet with a 

 similar situation. Their activity having ceased, 

 they become permanently fixed, and are then 

 about the size of the head of a very small pin. 

 They appear to affix themselves to their rest- 

 ing place by means of one of the little suckers 

 with which their anterior extremity is fur- 

 nished. 



The larva has now lost all power of loco- 

 motion, and quickly undergoes further changes 

 of form. The anterior extremity of the trunk 

 is widened, and the prolongations of the in- 

 ternal tunic quickly disappear. The central 

 portion of the tail becomes at the same time 

 empty, its contents being returned to the cen- 

 tral yolk-mass of the body. The sac or internal 

 tunic enclosing the yolk becomes much con- 

 tracted and spherical ; lastly, the yellow matter, 

 which was unequally divided, seems again to 

 be rearranged. The tail, which during the 

 early period of the existence of the larva per- 

 formed so important a part, being the only 

 instrument of locomotion, is now reduced to 

 its gelatinous or tegumentary portion ; and 

 this, after becoming more and more trans- 

 parent, withers, and finally is detached, or falls 

 away in shreds at a more advanced period of 

 the growth of the animal. The trunk, on the 

 other hand, is the seat of important and active 

 changes. The tegumentary portion of the 

 body is much widened, taking an oval out- 

 line, and is visibly augmented in bulk. The 

 interior tunic continues at first to lessen, and 

 becomes quite spherical, and many large 

 patches of a lighter yellow than the rest are 

 apparent, one of which occupies the anterior, 

 and two others the posterior portions of the 

 tunic {fig. 784. D). 



M. Milne-Edwards further remarks, that 

 the modifications already noticed ordinarily 

 occupy the space of from ten to twelve 

 hours ; and if the larva; are again examined 

 towards the end of the first day of their 

 sedentary existence, further changes in the 

 interior tunic may be observed. In a speci- 

 men carefully watched by him, the follow- 

 ing changes were noted. Instead of being 

 spherical, the large yellow sac became oval, 

 and its anterior part much thinned. It soon 

 afterwards again became elongated, and a 

 circular contraction divided it into two por- 

 tions {fig. 784-. E). The anterior portion, 

 smaller and lighter coloured than the pos- 

 terior, was rounded in front, and presented at 

 that part a large annular patch of a deep yel- 

 low, vaguely circumscribing a central paler 

 portion. The posterior part was swollen and 

 of a deeper yellow than the anterior, and quite 

 behind there was observable a minute patch 

 of a very clear yellow. This latter spot sub- 

 sequently became the heart, and the annular 

 spot on the other extremity of the body was 

 developed into the thorax of the animal. The 

 following day all these parts grew still more 

 distinct. The anterior portion of the inter- 



4 i 4 



