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Table 1 Summary and timeline of morphological changes during development and metamorphosis of Schizocardium californicum. Traditional staging terminology is copied from [14]

From: The development and metamorphosis of the indirect developing acorn worm Schizocardium californicum (Enteropneusta: Spengelidae)

Time

Ectoderm

Mesoderm

Endoderm

Traditional staging terminology

3–5 dpf

Circumoral ciliary band 2–4 serotonergic neurons in apical organ Apical tuft

Apical strand

Pharyngeal circular muscles

Protocoel, pore canal and hydropore

Tripartite gut

Müller stage

Larva just recognizable as a tornaria, longitudinal ciliary bands formed, telotroch not yet present

5–10 dpf

Eyespots Telotroch formation, neurotroch

Neuropil, dorsal axons

Anchoring muscles

 

Heider stage

Longitudinal ciliary bands without development of lobes and saddles; telotroch just formed.

10–18 dpf

Dorsal cluster of 5HT neurons in apical organ

Telotroch fully formed

  

18–22 dpf

Primary dorsal saddles and lobes

Ventral saddle

Addition of cell bodies in apical organ

  

Metschnikoff stage

Primary lobes and saddles in formation or formed.

Trunk coeloms not yet or having just appeared.

22–30 dpf

Apical tuft disappears

Primary ventral saddles and lobes

Lower dorsal lobe

Longitudinal muscles in pharynx

Mesocoels and metacoels

 

Krohn stage

Secondary lobes and saddles or tentacles formed or forming. Trunk coelom usually present. Collar coelom not yet present or just first appearing. Large size. The high point of larval development.

30–50 dpf

Secondary telotroch

Pulsatile vesicle

 

50–65 dpf

Serotonergic cell bodies throughout ectoderm

Well-developed serotonergic nerve net

Apical strand disappears

Mesocoels and metacoels increase in size

Muscle fibers in coeloms

Gill slits (5 pairs)

Spengel stage

Secondary lobes and saddles in regression or regressed. Collar and trunk coeloms present. Smaller than the previous stage, club shaped. Circular constriction about the middle of the body. Opaque.

0–5 h post onset of metamorphosis

Thickening of epidermis

Gill pore formation

Expansion of all coeloms (reduction of blastocoelar space)

Digestive tract retracts posteriorly

5–11 h post-onset of metamorphosis

Food grooves disappear

Circumoral ciliary band disappears

Protocoel replaces blastocoelar space in proboscis

Larval pharyngeal muscles disappear

Trunk musculature develops

Pharynx posterior to the mouth

Agassiz stage

Secondary lobes and saddles gone. Collar and trunk coeloms well developed. Regionalization indistinct.

Entire body more elongate than in previous stage. Analfield a conical bulge. Longitudinal ciliary bands shifted toward the apical plate, the gut shifted analward.

Protocoel very large.

12–24 h post-onset of metamorphosis

Epidermis opaque

Protocoel fills entire proboscis

Mesocoels replace blastocoelar space in collar

Well-developed longitudinal muscles in trunk

Growth of pharynx

Growth of gill slits

Stomach folds

24–48 h post-onset of metamorphosis

Resorption of telotroch

Transition from swimming to burrowing

Eyespots disappear

Metacoels replace blastocoelar

space in trunk

Additional gill slits

Metamorphosis stage

Metamorphosis begins at the end of larval life.

Proboscis, collar, and trunk regions delineated. Ciliary rows or tentacles in atrophy. Appearance of gill slits and buccal diverticulum.

In all germ layers: clear division of body into proboscis, collar and trunk. Trunk growth