Fig. 228. Advanced Erichthus larva of Squilla when the three posterior maxillipeds have become reduced to minute pouches. (From Claus.)
The most noteworthy points in the developmental history detailed above are the following:
(1) The thoracic and abdominal segments (apart from their appendages) develop successively from before backwards.
(2) The three last maxillipeds develop before the abdominal feet, as biramous appendages, but subsequently completely atrophy, and then sprout out again in their permanent form.
(3) The abdominal feet develop in succession from before backwards, and the whole series of them is fully formed before a trace of the appendages of the three hindermost thoracic segments has appeared. It may be mentioned as a point of some importance that the Zoæa of Squilla has an elongated many-chambered heart, and not the short compact heart usually found in the Zoæa.
The younger stages of the Alima larva are not known[193], but the earliest stage observed is remarkable for presenting no trace of the three posterior pairs of maxillipeds, or of the three following pairs of thoracic appendages. The segments belonging to these appendages are however well developed. The tail has its full complement of segments with the normal number of well-developed swimming feet. The larva represents in fact the stage of the Erichthus larva when the three posterior pairs of maxillipeds have undergone atrophy; but it is probable that these appendages never become developed in this form of larva.
Apart from the above peculiarities the Alima form of larva closely resembles the Erichthus form.
Nebaliadæ. The development of Nebalia is abbreviated, but from Metschnikoff’s figures[194] may be seen to resemble closely that of Mysis. The abdomen has comparatively little yolk, and is bent over the ventral surface of the thorax. There is in the egg a Nauplius stage with three appendages, and subsequently a stage with the Zoæa appendages.
The larva when it leaves the egg has the majority of its appendages formed, but is still enveloped in a larval skin, and like Mysis bends its abdomen towards the dorsal side. When the larva is finally hatched it does not differ greatly from the adult.
Cumaceæ. The development of the Cumaceæ takes place for the most part within the egg, and has been shewn by Dohrn (No. [496]) to resemble in many points that of the Isopods. A dorsal organ is present, and a fold is formed immediately behind this which gives to the embryo a dorsal flexure. Both of these features are eminently characteristic of the Isopoda.