Fig. 34. Embryo of a Squilla, magnified. a. heart.
Fig. 35. Older larva (Zoëa) of a Stomapod, magnified.
Of the two larval forms at present known which are with certainty to be ascribed, if not to Squilla, at least to a Stomapod, I pass over the younger one[[7]] as its limbs cannot be positively interpreted, and will only mention that in it the last three abdominal segments are still destitute of appendages. The older larva (Fig. 35), which resembles the mature Squilla especially in the structure of the great raptorial feet and of the preceding pair, still wants the six pairs of feet following the raptorial feet. The corresponding body-segments are already well developed, an unpaired eye is still present, the anterior antennæ are already biramose, whilst the flagellum is wanting in the posterior, and the mandibles are destitute of palpi; the four anterior abdominal segments bear biramose natatory feet, without branchiæ; the fifth abdominal segment has no appendages, and this is also the case with the tail, which still appears as a simple lamina, fringed on the hinder margin with numerous short teeth. It is evident that the larva stands essentially in the grade of Zoëa.
[1] Authorities are cited only for facts which I have had no opportunity of confirming.
[2] Bell (‘Brit. Stalk-eyed Crust.’ p. xlv) considers himself justified in “eliminating” Thompson’s observation at once, because he could only have examined ovigerous females preserved in alcohol. But any one who had paid so much attention as Thompson to the development of these animals, must have been well able to decide with certainty upon eggs, if not too far from maturity or badly preserved, whether a Zoëa would be produced from them. Moreover, the mode of life of the Land-Crabs is in favour of Thompson. “Once in the year,” says Troschel’s ‘Handbuch der Zoologie,’ “they migrate in great crowds to the sea in order to deposit their eggs, and afterwards return much exhausted towards their dwelling places, which are reached only by a few.” For what purpose would be these destructive migrations in species whose young quit the egg and the mother as terrestrial animals?
[3] In a memoir on the metamorphoses of the Porcellanæ I have erroneously described this appendage as the “flagellum.”
[4] Glaucothoë Peronii, M.-Edw., may be a young and still symmetrical Pagurus of this kind.
[5] The oldest observed larvæ (see Fig. 33) are characterised by the extraordinary length of the flagella of the outer antennæ, and in this respect resemble the larva of Sergestes found by Claus near Messina (Zeitschr. für Wiss. Zool. Bd. xiii. Taf. 27, Fig. 14). This unusual length of the antennæ leads to the supposition that they belong to our commonest Prawn, which is very frequently eaten, and is most nearly allied to Penëus setiferus of Florida. Claus’s Acanthosoma (l. c. Fig. 13) is like the younger Mysis-form of the larva figured by me in the ‘Archiv für Naturgeschichte,’ 1836, Taf. 2, Fig. 18, and which I am inclined to refer to Sicyonia carinata.
[6] Van Beneden, who regards the eye-peduncles as limbs, cannot however avoid remarking upon Mysis: “Ce pédicule n’apparaît aucunement comme les autres appendices, et paraît avoir une autre valeur morphologique.”
[7] ‘Archiv für Naturgeschichte’ 1863. Taf. 1.