LECTURES ON EMBRYOLOGY. 



therefore, here, series of eyes all around the mantle. 

 We have even the series nearly as completely de- 

 veloped, as in the Pecten, and we have a fully de- 

 veloped eye on the anterior part of the head, on 

 ihe side of whicb, there is one larger tentacle ob- 

 served ; making the analogy perfect. 



But let the lateral radijTientary eyes disappear 

 and the anterior pair remain, and we have the 

 ordinary condition of Gasteropoda^ so that the 

 question whether there is any similarity between the 

 Acephala and other Mollusks, must be answered 

 toy the assertion that the analogy is as complete as 

 ean ever be expected between animals of the same 

 great department, but belonging to different classes. 

 Endeed, in tracing the differences between the man- 

 tle of Margarita, (Plate XLIII,) and that of the 

 Acephala, we notice the anterior part of the mantle 

 has larger fringes corresponding to the region 

 where those larger eyes occur. So that we have an 

 uninterrupted series from these in which there are 

 yes all around, gradually to those which have 

 eyes only a part of the way round,, and to those 

 which have only two eyes. Tracing, however, this 

 structure further down, we come from Pecten to 

 shells, as in Mya, where there are no eyes at all. But 

 even in these, there are colored specks at the open- 

 ings of the mantle. So that we have a natural ap- 

 paratus with compound eyes, with perfect tensesjn 

 one order of Mollusca, as they exist in vertebrata, 

 down to those which have eyes with a rudimentary 

 crystalline lens, and still further down to those 

 specks which can enable the animal hardly ,if at all, 

 to distinguish between light and darkness. 



Here we have a new species of a so-called soft 

 shelled Clam, (Ascidia) (PlateXLI,) in which the 

 animal is included within a sac, and leaving only 

 two openings at one end. Now on the ends of 

 these openings we have ia tkis a new species, 



[PLATE XLI ASCIDIA OR SOFT-SHELLED CLAM.] 



Ascidia scutella which I have observed recently 

 in New Bedford colored dots, What are they 1 

 The last indication of the lowest condition of eyes 

 on the margin of those tubes, through which water 

 is introduced into the body, And through these, 

 and through the open tubes of Clams, we pass 

 gradually to those more complicated organs, as 

 they are seen in the higher species,with a pair of 

 eyes. From those in which we have eyes, to those 

 in which we have only colored dots, we have grad- 

 ual steps. 



And in this way from the most regular Cephalo- 

 poda (Plate XXXVI, fig. A.) down to the Ace- 

 phala, (Plates XXXV, XLIV and XLI) we have 

 the multiplication of these organs, tending to 

 transform well-defined organs into single colored 

 specks. 



In my next lecture I shall say a few words more 

 upon the structure of Moilusca, and then proceed 

 to illustrate their embryonic growth. 



LECTURE XI 



En every type of the animal kingdom, there have 

 been some forms observed which have perplexed 

 Naturalists, and whose natural positions have not 

 been ascertained until after extensive investiga- 

 tions. You remember with what difliculties we 

 struggled when examining the natural circumscrip- 

 tion of the type of Radiata^ how, many animals, 

 which had been considered as Polypi, had to be 

 excluded from that class, as it must be circum- 

 scribed by the observations of modern investiga- 

 tor?. Among Articulata, we felt the same difficul- 



ties, owing to the peculiar structure of many par* 

 asitic Worms, of many parasitic Crustacea, which, 

 when full grown, differ so widely from their em- 

 bryonic condition,that they cannot be arranged with 

 them, unless the whole history of their metamor- 

 phoses be ascertained by embryonic investigations. 

 The same difficulty occurs with Mollusks. 



If we had only to deal with animals with bivalve 

 shells, with the Snail-like Gasteropoda, or with the 

 Cuttle-fishes, as I showed in my last lecture, the 

 general structure could be traced in their outlines 1 . 



