In one series of larvae, probably not of Squilla itself, but of related genera, the young hatch out as “Erichthoidina” (Fig. [99]), with the thoracic appendages developed as biramous organs as far as the fifth pair, and with a single abdominal pair of limbs.
Fig. [99].—Erichthoidina larva of a Stomatopod, with five pairs of maxillipedes, and the first pair of abdominal appendages, × 10. (From Balfour, after Claus.)
The abdominal series of limbs is next completed; the second thoracic limb assumes its adult raptorial structure, but the succeeding three limbs become greatly reduced and may entirely degenerate, leaving the posterior six thoracic segments without limbs.
Usually the anterior three pairs are only reduced, and then redevelop side by side with the small posterior limbs as they appear. This larva is then termed the “Erichthus” (Fig. [100]); but when they completely disappear the larva is called a “Pseudozoaea,” owing to its resemblance to the Zoaea stage of the Decapoda, which is also characterised by the suppressed development of the thoracic segments.
Fig. [100].—Older Erichthus larva, with six pairs of abdominal appendages, × 15. (From Balfour, after Claus.)
The so-called “Alima” larva of Squilla is also a Pseudozoaea, but it is apparently arrived at directly without the previous formation and degeneration of the anterior thoracic limbs, the larva hatching out from the egg in the Pseudozoaeal stage.
Fam. Squillidae.—Of the six known genera none extend into the cold subarctic seas; the majority are characteristic of the warm or tropical seas (Gonodactylus), some of the species having very wide ranges, e.g. G. chiragra, which is completely circumtropical, and appears to have entered the Mediterranean at some period, though it is very rare there.